Design Decoupling Copilot Provider from HTTPS proxy

[lukehinds]

With clients such as Cursor, windsurf and others also leveraging a HTTPS proxy like copilot, I fully expect there might be boilerplate we can extract from copilot and make available to multiple providers

[lukehinds]

#439 related

[jhrozek]

We started thinking about this with @ptelang given that this is a prereq to the Cursor work and in general will make our life easier.

OK to move to ready/in progress @lukehinds ?

[jhrozek]

Progress - this is now proposed at https://docs.google.com/document/d/1x1_FxYE7VDkuBx6Ui1I8ZNOy5dEhSLMq4_KrRshLirs/edit?tab=t.0#heading=h.5652xtol8cnf

[jhrozek]

@lukehinds @ptelang before we start the refactoring I spent today getting the http requests in shape to handle streamed requests which cursor uses for sending protobufs through http. I'm not done yet, but so far it seems promising. The idea is to first just get to a state where we can gather the whole streamed request, unpack it and then say "oh this is protobuf" and send it to target. The next step will be checking and unpacking the protobuf body.