feat: improve base URL configuration

[adityakrmishra]

Addresses #5

Hi team,

This PR simplifies the client configuration by allowing users to provide a single BaseURL from which the Web, APIv3, and APIv4 URLs are automatically derived, as outlined in the original issue.

Key Changes:

• Centralized Parsing: Added a URLs struct and ParseURLs logic (urls.go) that intelligently routes between public github.com endpoints and GitHub Enterprise endpoints.
• Backward Compatibility: Updated Config to support BaseURL via YAML/JSON/Env, but existing explicit fields (WebURL, V3APIURL, V4APIURL) are preserved and take precedence if defined.
• Refactored Creators: Updated NewDefaultCachingClientCreator to utilize the new config.GetURLs() method.
• Testing: Added comprehensive tests (urls_test.go) covering public GitHub, Enterprise URLs (with and without schemes/stray paths), and the fallback precedence logic.

The code adheres to the existing project style, and all tests pass cleanly. Let me know if you'd like any adjustments to the enterprise URL parsing logic!

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feat: improve base URL configuration

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