libostree: Make OstreeRemote a public and internal API

libostree: Make OstreeRemote a public and internal API

Previously it was static to ostree-repo.c. Make it usable throughout
libostree so it can be used by an upcoming commit, but also expose the
typedef and reference counting functions so that opaque OstreeRemote
pointers can be used by user code, in anticipation of exposing more of
its API publicly in future.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #832
Approved by: cgwalters