feat: allow generating an ssh deploy key via the git plugin

While this might not seem super intuitive, the key is mostly related to git management, not the ssh-keys we currently use to add/remove access to push to dokku.

plugins/git/subcommands/generate-deploy-key

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+source "$PLUGIN_AVAILABLE_PATH/git/internal-functions"
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+cmd-git-generate-deploy-key "$@"