feat: search_messages

[uipreliga]

This pull request adds the ability to search for Outlook messages by a specific term, improves header handling for authenticated requests, and introduces a configuration option for specifying a search term. The main logic now conditionally uses the search feature if a search term is provided, otherwise it retrieves inbox messages as before.

New search functionality:

• Added a new search_messages method to the OutlookClient class, allowing searching for messages containing a specific term, with support for result limiting and field selection. The method also sorts results by receivedDateTime in descending order.

Configuration and main flow updates:

• Added a search_term option to the configuration, loaded from the SEARCH_TERM environment variable.
• Updated the main logic in outlook_main.py to use the new search functionality when a search term is specified, otherwise defaulting to inbox retrieval.

Request handling improvements:

• Enhanced the _make_request method in OutlookClient to accept custom headers, enabling use of the ConsistencyLevel header required for search queries.
• Improved token refresh logic to update the request headers more robustly when retrying after authentication errors.