Add support for push options

[naxxfish]

Git supports Push options which allow options to be sent along with the push.

These are received by a server side pre-receive and post-receive hooks -and some Git hosts provide useful functionality out of the box using these options, like skipping CI or setting variables for CI pipelines in Gitlab. They can also be useful when you have server side hooks which support options.

These are specified as a string, where the interpretation of that string is up to the receiver to decide how to interpret it. Often it's just a flag (e.g. ci.skip) or can be something with it's own parameter (e.g. variable=value).

On the command line, a user could use these options as such:

git push -o mypushoption

Most users who would use these options will have a set of options they are likely to use frequently - so for the UI it could be a free text entry box which has a dropdown of previous strings which were used successfully before.

[DanPristupov]

You can create such push using a branch custom command.