[FEATURE] Support custom gitlab instance

[hlidotbe]

Description

Hi,

I'm trying gk but I can't add a custom gitlab instance and it doesn't seems to use the one configured through the web interface.

Side-bug: gk assumes it can always launch a browser making it difficult to use behind SSH.

GitKraken CLI Version

2.1.1

OS Version

Ubuntu 24.04 amd64

Git Version

git version 2.43.0

Logs, Screenshots, Screen Captures, URLs, etc

No response