http3: respect request context while reading from the response body

[marten-seemann]

The Go documentation for the request context says:

For an outgoing client request, the context controls the entire lifetime of a request and its response: obtaining a connection, sending the request, and reading the response headers and body.

The http.Response.Body therefore needs to return the context error as well. I confirmed that this is what HTTP/2 does.

Currently, we don't even pass the context to the body, and it's not clear to me how to correctly respect the context without spawning a new Go routine, since the Stream.Read call is blocking call.

Originally posted by @marten-seemann in #4203 (comment)

[marten-seemann]

@jhump provided a test case in #4203 (comment) that we can use without any modification.