feat: fixes setting of authentication realm url.

[marcosuma]

Description

When authenticating to App Service (Realm) there is a bug that impedes using a base_url different than realm.mongodb.com. As such, using realm-dev is not possible. With this fix, we are able to successfully authenticate. This is not an external bug.

Link to any related issue(s): https://jira.mongodb.org/browse/INTMDB-1010

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	12%	❌
Summary	3% (271 / 10310)	❌