fix: remove validation of empty public_key and private_key attributes in provider config to avoid breaking change

[AgustinBettati]

Description

Related tickets: INTMDB-999 (manual testing of provider configuration), INTMDB-971 (migration to framework provider, which included changes to the provider configuration)

During manual testing of the provider configuration using AWS Secrets Manager Authentication method, the following valid case was tested:

provider "mongodbatlas" {
  base_url = "https://cloud-dev.mongodb.com/"
  assume_role {
    role_arn = "arn:aws:iam::358363220050:role/agustin-role-for-sts"
  }
  secret_name           = "testing-terraform-provider-with-aws-sm"
  region                = "eu-north-1"
  aws_access_key_id     = "<access key>"
  aws_secret_access_key = "<secret access key>"
  aws_session_token     = "<sts session token>"
  sts_endpoint          = "https://sts.eu-north-1.amazonaws.com/"
}

Note: no environment variables defined.

• In 1.11.0 this provider configuration correctly retrieves credentials stored in AWS Secret Manager and following resource definitions work as expected.
• In our current migration branch (CLOUDP-189585-plugin-framework-migration) this case results in an error Error: attribute public_key must be set

Initially we defined validations for both of these attributes as we can see they are defined as required. However, since a default function is provided which will always return a value (either value from environment variables or simply empty string) these attributes can be left empty allowing support for cases like the above.

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[andreaangiolillo]

LGTM. left one question that is not blocking. Thanks for fixing this bug

[andreaangiolillo]

LGTM. Thanks for the follow up changes 💯

[maastha]

LGTM, nice one!