copliot: Add an extension point for credentials resolution

[ScottGuymer]

#1244

Hey, I just made a Pull Request!

#1244

Added in an extension point for the copilot backend plugin to allow for custom resolution of credentials for the GitHub Enterprise Copilot API

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
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[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-copilot-backend	workspaces/copilot/plugins/copilot-backend	minor	v0.1.2

[ScottGuymer]

I think this needs to change to work for most people.

For most users of github they have github.com as the host in their integrations config. The code here means that even if they configure github.com in the copilot section it will try to resolve api.github.com and fail (most people would not have this configured in their config)

So i thin kthis needs to be url: host to make it work for most people.

There is also the issue that the integrations config can be done with a github app which would not have access to the enterprise API at all. This is how we have it configured and probably most users of GH will use an app (I'm guessing).

I think that the default here could be to have a specific token in the copilot section as this token is specific to accessing the enterprise API.

@awanlin i know you suggested this change here but im not sure it works in this instance as this enterprise API is a bit specific and i dont think enterprise APIs are used anywhere else that i have seen in backstage (yet).