ostree: Improve formatting for well-known summary metadata keys

ostree: Improve formatting for well-known summary metadata keys

If a summary metadata key is well-known, like ostree.static-deltas, or
ostree.summary.last-modified, format it a little more nicely.

This is especially important for timestamps like last-modified, since
otherwise they’re formatted as a big-endian uint64, which is basically
unusable for the user.

Non-formatted output can still be retrieved using the OSTREE_DUMP_RAW
flag, and the non-formatted key name is always printed for clarity.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #826
Approved by: cgwalters