Move SoupRequester API into SoupSession, declare SoupRequest stable

Move SoupRequester API into SoupSession, declare SoupRequest stable

Add soup_session_request() and soup_session_request_uri(),
implementing basically the same behavior as soup_requester_request()
and soup_requester_request_uri() (but without requiring a separate
SoupSessionFeature), and remove the unstable-api ifdefs from
soup-request*.h.

SoupRequester still exists, but it is still guarded by the
unstable-api ifdefs, and is just a dummy wrapper around the
functionality that is now in SoupSession.