Add support for github enterprise instance (GHEC), small improvements to error handling on authentication, logging

[simchri]

• Support enterprise github instances, by making the github url(s) configurable
• treat authentication failure and failure to retrieve model list as critical errors (results in clearer error message in the chat pane; previously "model ... not found" now "Failed to authenticate" etc.)

[deathbeam]

The very important thing to remember is that the plugin is not actually tied to github copilot in any way outside of the github copilot provider. So adding configuration url for it anywhere else is not really an option. I can look at this later maybe or resurrect the other PR that is open for a while after I figure out some nicer api for it

[simchri]

I see that it's a bit of a design-impasse. I am aware that the approach that fits the current design would have been to implement a "ghec provider". However, this provider would be very similar to the default GitHub provider (a ton of duplication of logic in user's config)

On the other hand, making the default provider configurable affects the reusability of its components for custom providers, as the behavior now depends on the config.