Add _core module with shared sync/async client using iter_coroutine

[scotttrinh]

This is not the final form (we want pydantic models, etc), but this is the POC for using the iter_coroutine approach we discussed. How does this feel?

[vercel]

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	respx@​0.22.0

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[1st1]

Suggested change

	async def close(self) -> None: ...
	async def close(self) -> None:
	raise NotImplementedError

[1st1]

Suggested change

	) -> httpx.Response: ...
	) -> httpx.Response: ...
	raise NotImplementedError

[1st1]

Suggested change

	class AsyncVercelClient:
	class AsyncIOVercelClient:

and we also need to add a TrioVercelClient

[1st1]

Aside from a couple of nits this is exactly where I wanted to go with this. Please push forward. I'd add "trio" support + tests and once we are there, I'd start converting the code + increasing the API coverage with those three targets (asycio, trio, blocking io) in mind.