lib/repo-finder: Add mount based OstreeRepoFinder implementation

lib/repo-finder: Add mount based OstreeRepoFinder implementation

This is a basic implementation of OstreeRepoFinder which resolves ref
names to remote URIs by looking for them on any currently mounted
removable storage volumes. The idea is to support OS and app updates via
USB stick.

Unit tests are included.

This bumps libostree’s maximum GLib dependency from 2.44 to 2.50 for
g_drive_is_removable(). If GLib 2.50 is not available, the call which
needs it will be omitted and the OstreeRepoFinderMount implementation
will scan all volumes (not just removable ones); this is a performance
hit, but not a functionality hit.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #924
Approved by: cgwalters