feat: Notifications submodule buckets ARN generation in alternate partitions (e.g. GovCloud)

[tculp]

Is your request related to a problem? Please describe.

The notifications submodule generates the bucket ARN with

locals {
  bucket_arn = coalesce(var.bucket_arn, "arn:aws:s3:::${var.bucket}")
...

However, this does not work when the partition of the ARN is not aws, e.g. in GovCloud.

Describe the solution you'd like.

The module should use the aws_partition data source to pull the current provider's partition.

Describe alternatives you've considered.

Specifying the bucket ARN directly.

This is strange because you then need to specify both the bucket and bucket_arn variables, since neither is computed from the other, and both are used in different resources.

[tculp]

I noticed after creating this feature request that the bucket_arn variable also exists, so I updated the request accordingly.

[github-actions]

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