Expose identifier in order to work with AWS provider v5

[simonrondelez]

Description

Needed to mitigate https://github.com/hashicorp/terraform-provider-aws/blob/main/CHANGELOG.md#500-may-25-2023:

resource/aws_db_instance: The change of what id is, namely, a DBI Resource ID now versus DB Identifier previously, has far-reaching consequences. Configurations that refer to, for example, aws_db_instance.example.id will now have errors and must be changed to use identifier instead, for example, aws_db_instance.example.identifier (hashicorp/terraform-provider-aws#31232)

Related Issue

Motivation and Context

How Has This Been Tested?

manually

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have followed the PR process as described here
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.