Fix/payment link autocorrect

[deadtik]

Description

Fixes #321.

Users frequently pass string values (e.g., "true", "false") in the options dictionary, expecting them to work like booleans. The Razorpay API strictly requires integers (1/0). When strings are sent, the API silently ignores them (defaulting to False), causing issues like PayPal options not appearing.

Changes

• Added input sanitization to payment_link.create.
• The method now detects string values ("true", "false") in options.checkout.method and automatically casts them to integers (1, 0).
• Updated docstrings and added type hinting (PEP-8 cleanup).

Verification

• Verified locally that passing {"card": "true"} is correctly transformed to {"card": 1} before the network request.