chore: CLOUDP-214958: Run acc tests faster in local reusing cluster database_user

[andreaangiolillo]

Description

Ticket: CLOUDP-214958

This PR adds the option to use a personal cluster to run the acceptance tests locally

• 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 checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• 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

Testing

Tested locally

With my cluster

Running tool: /Users/andrea.angiolillo/.asdf/shims/go test -timeout 300000s -run ^TestAccConfigRSDatabaseUser_withScopes$ github.com/mongodb/terraform-provider-mongodbatlas/internal/service/databaseuser

ok  	github.com/mongodb/terraform-provider-mongodbatlas/internal/service/databaseuser	12.494s

Without cluster

2023-12-19T15:38:03.474Z [DEBUG] sdk.helper_resource: Finished TestCase: test_name=TestAccConfigRSDatabaseUser_withScopesAndEmpty
PASS