Feature/ssh keys new

[ericb-summit]

This commit enables ssh authentication for ssh-based submodules by

1. firing up ssh-agent on /tmp/ssh-agent.sock
2. adding the keys (passphrases not supported)
3. setting SSH_AUTH_SOCK to /tmp/ssh-agent.sock
4. setting git core.sshCommand such that host keys are ignored

[ericb-summit]

Okay this is the last delta I have over master HEAD.

[samcontesse]

This is adding a dependency on openssh/ssh-agent which is not nice. As this does not impact the rest of the feature, let's merge it and improve it later. I guess this would be needed only when submodules are not on the same server and relative submodules cannot be used.

[ericb-summit]

Correct.

For the future, we could: https://pkg.go.dev/golang.org/x/crypto/ssh/agent