ssh signing: add ssh key format and signing code

ssh signing: add ssh key format and signing code

Implements the actual sign_buffer_ssh operation and move some shared
cleanup code into a strbuf function

Set gpg.format = ssh and user.signingkey to either a ssh public key
string (like from an authorized_keys file), or a ssh key file.
If the key file or the config value itself contains only a public key
then the private key needs to be available via ssh-agent.

gpg.ssh.program can be set to an alternative location of ssh-keygen.
A somewhat recent openssh version (8.2p1+) of ssh-keygen is needed for
this feature. Since only ssh-keygen is needed it can this way be
installed seperately without upgrading your system openssh packages.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>