Add API usage option to telegram-phone-number-checker with FastAPI support

[GGontijo]

Hi! I'm excited to contribute to the telegram-phone-number-checker project and to collaborate with the Bellingcat team, whose projects I really admire!

In this PR, I've added the option to run the tool as an API using FastAPI. This change allows users to check phone numbers associated with Telegram via an API interface, in addition to the traditional CLI usage.

Changes made:

• Implemented FastAPI with a lifespan method to manage the Telegram session (connect on startup and disconnect on shutdown).
• Added a --use-api option to start the FastAPI server.
• Updated the README with a new section explaining API usage, including details on the /get_user_info endpoint.
• The phone number validation via CLI remains unchanged and fully functional.

Testing:

I performed tests to ensure the tool works seamlessly both via CLI and API. I'll include a screenshot below showcasing the API test in action.

I'm really excited to see this feature integrated and hope it helps extend the tool's usability!

Closes #7.