Add HTTPRequestStateMachine (#386)

This commit adds an explicit `HTTPRequestStateMachine`. It is intended to be used in two instances in the future:

1. As a substate machine for a `HTTP1ConnectionStateMachine`, when the connection is in a request.
2. As a state machine for a `HTTP2RequestHandler` that operates on a single http2 stream created by the multiplexer.

The new code paths are not triggered today, but will be enabled in the future.

Author: fabianfett