Add configurable GitHub Enterprise API base URL for key checks and API calls

[SuyashJain17]

Description

• This PR adds support for GitHub Enterprise API endpoints by making the GitHub API base URL configurable.
• A new github_api_base_url option is introduced under the Keys configuration section, defaulting to the public GitHub API.
• All hardcoded https://api.github.com references across the codebase have been replaced with calls to a centralized URL management utility.

This PR fixes #3277

Summary
This implementation addresses issue #3277 by:

• Adding configurability – Users can now specify their GitHub Enterprise API URL via the github_api_base_url config option.
• Centralizing URL management – All GitHub API URLs are constructed through a single utility module.
• Maintaining backward compatibility – The default value remains https://api.github.com, so existing installations work without any changes.
• Comprehensive coverage – Updated all 17 files that previously contained hardcoded GitHub API URLs, including REST, GraphQL, rate-limit, and worker endpoints.

Notes for Reviewers

• The change is fully backward compatible and opt-in for GitHub Enterprise users.
• The centralized URL utility ensures consistent API URL construction across tasks, workers, and key validation.
• Some GitHub Enterprise installations may require /api/v3 in the base URL; users can provide the full API root via configuration as needed.

Signed commits

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