SSHAgentKMS docs

[tashian]

step-ca supports SSHAgentKMS (for SSH keys only; not for X.509), but we don't have a section in the Configuration docs for it. Here's the previous documentation, which needs to be fleshed out:

SSHAgentKMS

SSHAgentKMS is a KMS that wraps a ssh-agent which has access to the keys to
sign ssh certificates. This was primarily written to be able to use gpg-agent
to provide the keys stored in a YubiKeys openpgp interface.

{
    "kms": {
        "type": "sshagentkms"
    },
    "ssh": {
        "hostKey": "sshagentkms:cardno:000123456789",
        "userKey": "sshagentkms:cardno:000123456789",
    },
    ...
}

This KMS requires that "root", "crt" and "key" are stored in plain files, as for
SoftKMS.