Consider adding a MockStream

[lundberg]

It might be of use if RESPX had a MockStream utility that could be passed as stream when creating a httpx.Response.

The MockStream should suport both sync and async contexts.

Ideas...

stream = MockStream(json={})
stream = MockStream(text="hello")

Also, mocking a response stream with content from local file would be of interest in some cases, e.g. large test responses. It should probably take either a file-path string or a file-like object.

stream = MockStream(file=...)

Other things to consider is form data and multipart responses.