feat: add desktop authenticators methods to client.Admin

[MarkTripod-Duo]

Tested-by: new test case in tests/admin/test_desktop_authenticators.py

Description

Added the following new methods to support usage of the /admin/v1/desktop_authenticators endpoint

• get_desktop_authenticators_generator()
• get_desktop_authenticators()
• get_desktop_authenticator_by_key()
• delete_desktop_authenticator()

Motivation and Context

Added new methods to maintain consistency with current Admin API functionality and documentation.

How Has This Been Tested?

New tests added via tests/admin/test_desktop_authenticators.py

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)