Adding/removing depots

[ayushuk]

Along with the changes to depos in branchline, we want to explore the possibilities of allowing users to add their own custom depos containing their choice of templates following the template JSON format, it is up to the user to follow the depo standard as this feature should assume it is the same format as pros-mainline.

This feature should be a new conductor command that has users pass in a name and URL to the depo JSON similar to:
pros conductor add-depot/remove-depot/query-depots.

Note that this is a experimental feature that may not be used by many users and not be implemented alongside branchline.