Adds support for a configurable path where ssh network info stats
are going to be saved.

Remnant from the ssh background connection support

This patch provides UI support for the network inf dir where
ssh metrics will be saved. Users can configure the default
location for this place.

Also, the default directory is renamed to ssh-network-metrics
for a better description.

Toolbox spawns a native process running the SSH client. The ssh client then spawns
another process which associated to the coder proxy command. SSH network metrics
are saved into json file with the name equal to the pid of the ssh command (not
to be confused with the proxy command's name)

When ssh is connected a poll job is created that searches for the
SSH command pid, and then determines the network metrics json file.

The pid can change multiple times in a Toolbox session, especially
when the OS goes to sleep and comes back - the ssh is respawned.
This is the reason we don't cache the pid, instead always search
for it.

The poll job activates every 5 settings and it is cancelled when
the user stops the ssh connection.

This commit contains the data class necessary to
unmarshall the ssh network metrics and also the
necessary moshi plumbing for converting the json file.