CLOUDP-319308: Add LDAP support to AtlasDatabaseUser

[tvdw-mongodb]

Summary

This adds support for LDAP for Atlas Database Users.

Proof of Work

Added unit test

Checklist

• Have you linked a jira ticket and/or is the ticket in the title?
• Have you checked whether your jira ticket required DOCSP changes?
• Have you checked for release_note changes?
• Have you signed our CLA?

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