Forward ssh-agent socket into the sandbox

[mzabaluev]

To facilitate working with git over ssh, it would be nice if the sandbox had access to the ssh-agent socket from the user session. flatpak/flatpak#1438 says this is feasible with manual override options of flatpak run, but it would be great if the app had it out of the box.

[mzabaluev]

This could alleviate #23 as well: in my usage, I'm mostly using keys enrolled into the session-provided agent.

[JeffLabonte]

I agree with @mzabaluev, that would great to be able to do so out of the box!

[nedrichards]

This requires at least flatpak 0.10.3 (releases before then fail when they see unknown permissions). We only have flathub version stats from Flatpak 0.11.3 onwards, and that or newer is currently at 60% of usage. I'd love to see that get higher before we turn this on by default. (we believe the main 'offender' is CentOS/RHEL which are on 0.8.8 fwiw.)

[mzabaluev]

For an individual installation, the problem can be solved persistently with
flatpak override --socket=ssh-auth com.visualstudio.code.
So only "out of the box" part is missing.

[nedrichards]

Should be fixed by 4d6ea9a