provider/aws: Make it possible to remove S3 bucket policy

[radeksimko]

Due to a bug introduced in #8615 it is impossible to remove a policy as the policy field is marked as Computed and "existing-policy" => "" is always a no-op if the field is Computed.

There's actually already an acceptance test covering this and it was failing for the past 15 days after merging #8615 , just returning a slightly misleading error message - so I fixed that too in this PR.

I understand the reason for making this field Computed was:

1. to expose policy which would be added/managed via aws_s3_bucket_policy after refresh
2. to prevent aws_s3_bucket from deleting the policy which was set in the context of aws_s3_bucket_policy - i.e. prevent those resources from fighting with each other
3. to allow import of bucket with policy

(2) can be still achieved without Computed - see my third commit.
(3) can be still achieved without Computed - see my last commit of this PR

I remember @stack72 and I discussed recently that there may be some plans in regards to overlapping/nested resources like these - but I don't know what these are - if there are any.

Either way this is now broken, so I think it needs fixing.

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=AWSS3Bucket'

[jen20]

Hi @radeksimko! At first blush this looks like a good set of fixes, I'll review the this morning and get them merged.

[jen20]

These changes look good to me. There are currently acceptance test failures on:

• TestAccAWSS3BucketNotification_importBasic
• TestAccAWSS3Bucket_Notification
• TestAccAWSS3Bucket_WebsiteRoutingRules
• TestAccAWSS3Bucket_Lifecycle

However, none of these seem related, and fail in the same manner without these commits.

[ghost]

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