Cherry-pick #15668 to 7.x: Support application default credentials (ADC) for Google Pub/Sub

[sayden]

Cherry-pick of PR #15668 to 7.x branch. Original message:

What does this PR do?

Update the Google Pub/Sub input to support reading Application Default Credentials
(ADC) in addition to the credentials_file and credentials_json config options.

If neither config option is set then it will attempt to search for the default credentials.
Generally this means reading the GOOGLE_APPLICATION_CREDENTIALS environment
variable plus searching a few other well known locations.

The googlecloud module was updates to support all three authentication mechanisms.

Why is it important?

Without this change, Filebeat requires a path to a file with credentials to be provided in its config and is unable to use for example a default service account.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

1. Upload the Filebeat binary to a GCP instance created with default config
2. Run the binary configured to read from PubSub, but do not provide any credentials
3. Filebeat should be able to detect that it's on GCP and use ADC (get a token for the default service account for the VM from the metadata server)

Related issues

• Closes Filebeat doesn't use ADC on GCP instances #15667

Use cases

Sending logs to Elastic from PubSub queue

Screenshots

Logs