Refactor copilot provider to allow cursor support

[jhrozek]

Currently the copilot provider mixes copilot and general network proxy functionality. If we want to add cursor support in a reasonable way, we need to sptlt those. The design was tracked in #384

[jhrozek]

We're parking the cursor support for now, the one thing we might want to salvage out of this is the RequestState which parses the request into headers, body and handles chunking. OTOH we haven't really had issues with the cursor provider lately, so might as well park it until we need to do any changes to the copilot provider.