Add SQL Backup Run Datasource

[c2thorn]

Part of hashicorp/terraform-provider-google#2446

Backup Run datasource is essential for grabbing a most recent backup run ID to restore to.
https://cloud.google.com/sql/docs/mysql/admin-api/rest/v1beta4/backupRuns

most_recent field behavior inspired by the AWS equivalent https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/db_snapshot#most_recent

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

`google_sql_backup_run`

[c2thorn]

Requesting sanity check on using backup_id vs name for the id field in https://cloud.google.com/sql/docs/mysql/admin-api/rest/v1beta4/backupRuns#BackupRun

[slevenick]

Seems reasonable to me. We have similar things in compute, map_id and similar as substitutes for id.

Why would a user want to specify the backup_id if the point of this resource seems to be to retrieve the id of the most recent backup?

[c2thorn]

Really just for access to the Computed fields of the resource. Definitely not as useful without adding more of the fields to the datasource, but status and start_time may be useful to a user right now.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 311 insertions(+))
Terraform Beta: Diff ( 6 files changed, 311 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=164780"

[c2thorn]

For context, the AWS equivalent behavior is shown here: https://github.com/hashicorp/terraform-provider-aws/blob/master/aws/data_source_aws_db_snapshot.go#L161

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 311 insertions(+))
Terraform Beta: Diff ( 6 files changed, 311 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=164780"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceSqlBackupRun_notFound|TestAccDataSourceStorageBucketObjectContent_Basic|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccContainerCluster_withConfidentialNodes|TestAccContainerCluster_withPrivateClusterConfigMissingCidrBlock|TestAccContainerCluster_withIPv4Error|TestAccContainerNodePool_withKubeletConfig|TestAccDataprocJob_Spark You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=164789"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceSqlBackupRun_notFound|TestAccDataSourceStorageBucketObjectContent_Basic|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccContainerCluster_withConfidentialNodes|TestAccContainerCluster_withPrivateClusterConfigMissingCidrBlock|TestAccContainerCluster_withIPv4Error|TestAccContainerNodePool_withKubeletConfig|TestAccDataprocJob_Spark You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=164789"

[slevenick]

I'm not seeing TestAccDataSourceSqlBackupRun_basic run anywhere... Know why that might be?

[slevenick]

Seems reasonable to me. We have similar things in compute, map_id and similar as substitutes for id.

Why would a user want to specify the backup_id if the point of this resource seems to be to retrieve the id of the most recent backup?

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 315 insertions(+))
Terraform Beta: Diff ( 7 files changed, 316 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=164863"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceGoogleSubnetwork|TestAccDataSourceStorageBucketObjectContent_Basic|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccComputeFirewall_update|TestAccComputeFirewall_priority|TestAccComputeFirewall_disabled|TestAccComputeFirewall_enableLogging|TestAccContainerCluster_withConfidentialNodes|TestAccContainerCluster_withPrivateClusterConfigMissingCidrBlock|TestAccDataprocJob_Spark You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=164866"