Adds Subscriptions module for webhook notifications (#16)

* Configures proper test support file compilation

Replaces Code.require_file with idiomatic elixirc_paths configuration.
Mix now automatically compiles test/support files in test environment.

* Adds Subscriptions module for webhook notifications

Enables real-time change notifications from Microsoft Graph instead of
polling, reducing API calls and improving responsiveness for calendar
events, messages, and other resources.

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: johnnyt