lib/pull: Journal fetch results

lib/pull: Journal fetch results

This is a continuation of addition of journaling to libostree; see
e.g. <https://github.com/ostreedev/ostree/pull/708>.

I wanted more information at the end of fetches; in particular
some details about the delta execution (what opcodes etc.), but
this is a first step: we log things like the transferred data
as well as whether or not GPG was enabled, etc.

One awkward thing about this is how we map the fetcher options like
`tls-ca-path` back out into an enum for the code to log. But eh, hard to fix
without a bigger refactoring.

Closes: #1032
Approved by: jlebon