feat: set format for AWS region value in the provider definition

[marcosuma]

Description

When authenticating with an aws_role (see example below), the AWS region value must be lowercase and with - symbol. Considering that we allow to consume the region value also from environment variables and that it is generally allowed to have multiple ways to identify the region (e.g. in MongoDB Atlas we use the convention US_EAST_1 instead of us-east-1), this change make sure that the region value is always converted to its right format.

provider "mongodbatlas" {
  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     = "<temporary sts access key>"
  aws_secret_access_key = "<temporary sts secret access key>"
  aws_session_token     = "<temporary sts session token>"
  sts_endpoint          = "https://sts.eu-north-1.amazonaws.com/"
} 

Link to any related issue(s): https://jira.mongodb.org/browse/INTMDB-1034

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
• If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.

Further comments

[github-actions]

Package	Line Rate	Health
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas	2%	❌
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/framework/validator	68%	➖
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/util	17%	❌
Summary	3% (272 / 10318)	❌