Support for agentic copilot

[jhrozek]

The latest copilot iteration, currently in the insiders program brings agentic capabilities to copilot. There's a couple of things that need work on the codegate side, but mainly we need to finish getting rid of using litellm types inside the pipelines because the agentic mode relies heavily on tools which get mangled.

We also need to support additional endpoints that they use, so far we found out POST to api.github.com/chunks and POST /v1/engines/copilot-centralus-h100/speculation - route them through pipelines as usual.

[jhrozek]

This is being fixed in the remove-litellm branch.