Allow specifying a Git Repo URL to extend or customize Orbiter instead of a local override.py file

[odaneau-astro]

Allow specifying a Git Repo URL to extend or customize Orbiter instead of a local override.py file.

Having the ability to reference an override.py file located in a separate git repo would avoid back end forth when the override file is updated.
Most of the time, migration projects are done as a team with a single person handling the orbiter customization and multiple people brute-forcing the refactoring.

Using a central Git repo would simplify syncing the overrides across the team by avoiding pushing and pulling from Git when changes are made to the override file.