Add Copilot custom agents enterprise endpoints #4524

The Copilot custom agents API exposes endpoints to list enterprise custom agents and manage their source organization/repository, but go-github does not model them yet.

Endpoints:

• GET /enterprises/{enterprise}/copilot/custom-agents
• GET /enterprises/{enterprise}/copilot/custom-agents/source
• PUT /enterprises/{enterprise}/copilot/custom-agents/source
• DELETE /enterprises/{enterprise}/copilot/custom-agents/source

Docs: https://github.com/proxy/docs.github.com/en/enterprise-cloud@latest/rest/copilot/copilot-custom-agents

I plan to work on this.