Allow users to add their own mappings

[lukehinds]

Over time the mappings and going to become more complex and users may well want to add (or even remove (deny list style)) their own mappings.

https://github.com/stacklok/codegate/blob/main/src/codegate/providers/copilot/mapping.py should be either loaded as config, or even better moved to the database and be something that can be edited via the codegate dashboard

[lukehinds]

Has synergies with #454

[jtenniswood]

Would this be applied per provider?
Would this work on a workspace or global level?

[jhrozek]

This was just about the copilot provider mappings, not the pipeline. Moving back to backlog until we have a use-case.