RFC extended-remote

When selecting the option to connect to a remote gdbserver there is no way of selecting that you need an extended-remote connection. This is for example needed when debugging embedded targets with a blackmagic probe.

I'm considering implementing this in gdbgui in either one of two ways.
- Adding a checkbox when selecting server
- Adding a whole new connection target.

Imho adding a checkbox to the current server target is the better option, but this could get out of hand when there are even more settings to consider for either this or other ways of opening a debug connection. So the checkbox could also hide behind a dropdown where more options can be added.

The new target is the easier solution, but when more options need to be added this will make the list way too long.

I would like to hear your opinion on this, or if this is even something people besides me want in this project.