dir: Add logging

Write to the systemd journal for basic operations
that modify installations, such as pull, deploy,
uninstall.  Include some useful information as
separate fields, such as OPERATION, INSTALLATION,
REMOTE, REF, COMMIT.

We add the logging in the api layer that is used
by the system-helper as well, so that changes
from that side are logged as well.

All logs we write currently use the same message
ID, defined as FLATPAK_MESSAGE_ID in the headers,
for easy retrieval of messages.
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