Add an option to specify a custom SSH port

Hi! Enjoying the plugin so far, besides a few snags like no signing not a biggie, saw the other issue about it, and I don't mind committing from the command line.

On a similar note however, my repo is hosted on a forgejo instance on which ssh is running on a non-standard port (2221) so as to not interfere with other services running on the same host.

As far as I can see there is no way to configure the SSH port used by the plugin. As a result currently the plugin is serving as more of a window into my git repo's current state (still very useful) rather than letting me perform git operations without leaving Godot, but I do understand this is a work in progress.

Again this isn't a big deal for me, I'm already committing in the CLI so I may as just do my git operations there too.