balena-ssh: add -A flag to allow forwarding SSH agent

We need access to git ssh repositories from the balena-installed system. For security reasons we would like to do this without installing private keys on the balena-installed system.

Our solution is to forward the SSH agent while cloning/pulling these repositories with the -A flag for the ssh command, something the balena ssh command currently does not support.

Would it be possible to add the possible use of the -A argument to the balena ssh command?

More details on -A option in OpenSSH can be found on: https://linux.die.net/man/1/ssh