Add pagination functionality to FHIRClient and related unit tests

[LanaNYC]

PR Title: Add Pagination to FHIRClient with Unit Tests

Description:

This PR introduces pagination to FHIRClient, allowing efficient navigation of large FHIR datasets. Key methods include:

• fetch_next_page: Retrieves the next page via the next link in the FHIR Bundle.
• get_next_link: Extracts the next link from the Bundle's links.
• sanitize_next_link: Validates and sanitizes the next link URL.
• execute_pagination_request: Handles the HTTP request and manages errors when fetching the next page.

Testing:

• Unit tests in client_pagination_test.py cover pagination, handling missing next links, and errors.

Notes:

• Backward-compatible, no breaking changes.

Please review and provide feedback.

Fixes: #108