feat: adding bulk user validation endpoint for admin console

[jacobo-dominguez-wgu]

Description

Implements a new api/authz/v1/users/validate/ endpoint for bulk user validation to support admin console operations during role assignments.
Accepts a JSON payload with a users array containing usernames and/or email addresses, validates the list of input data using serializer and checks for required field, proper format, deduplication, and finally processes each identifier individually to check existence.
Required permissions: manage_library_team OR manage_course_team in any scope.
Superuser/staff bypass.

It resolves #232

Deadline: Verawood.

Testing instructions

Make sure to have a local tutor dev installation.
Only users with team manage permissions are allowed to use this endpoint.

POST http://local.openedx.io:8000/api/authz/v1/users/validate/

{
  "users": [
    "user.invalide@example.com", "duplicated_user", "duplicated_user", "any_valid_user"
  ]
}

Expected response:

{
  "valid_users": [
    "any_valid_user",
  ],
  "invalid_users": [
    "user.invalide@example.com",
    "duplicated_user",
  ],
  "summary": {
    "total": 3,
    "valid_count": 1,
    "invalid_count": 2
  }
}

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[rodmgwgu]

The functionality of this class looks good, however I'm not convinced on the "Global" name, as we use that term to refer to "*" scopes on other parts of authz.

Perhaps name it like "AnyScopePermission"?

[jacobo-dominguez-wgu]

It make sense, renamed to AnyScopePermission.