feat(copilot): adds github copilot login support

[nathabonfim59]

As mentioned in #426, currently, we require an external program to generate the Github Copilot oAuthToken.
This PR adds all necessary structures to do this first login within mods.

How to test

1. Backup and remove your oAuthToken (~/.config/github-copilot)
2. Remove the cached token (~/.local/share/mods/temp/)
3. Try to run a new generation using mods
4. Following the instructions using the DeviceCode authentication flow (it will show in your terminal)

It will automatically ask for login and poll for the authToken every 5 seconds (defined by the API) and perform the generation afterwards

PR Summary

Authentication Refactor:

• Added new constants for device code and token URLs and the client ID (copilot.go).
• Introduced new data structures for handling device code and token responses (CopilotDeviceCodeResponse, CopilotDeviceTokenResponse, CopilotFailedRequestResponse, copilotGithubOAuthTokenWrapper, copilotOAuthToken) (copilot.go).

New Functions:

• Added copilotLogin function to initiate device code authentication and fetch the OAuth token (copilot.go).
• Added fetchCopilotRefreshToken function to poll for the OAuth token using the device code (copilot.go).
• Added saveCopilotOAuthToken and copilotRegisterApp functions to save the OAuth token and register the app version (copilot.go).

Refactoring Existing Functions:

• Modified getCopilotRefreshToken to use the new copilotLogin function for obtaining the OAuth token (copilot.go).
• Updated getCopilotAccessToken to fetch the OAuth token instead of the refresh token (copilot.go).

[nathabonfim59]

Since it's no longer a single package, we can't access the Versions variable. What would be the best approach here?

Alternatively, we can ditch the "app registration" entirely. This will be only used by Microsoft to track how many users of a given application are using Copilot. But we're not sending this info in the request, so it could be removed without a big impact.

[caarlos0]

I wonder if we could use https://github.com/cli/oauth for this - or parts of it?

[nathabonfim59]

I wonder if we could use https://github.com/cli/oauth for this - or parts of it?

Thanks for the feedback! Using the github.com/cli/oauth library would certainly be more elegant than the current implementation.

I've done this "by hand" to avoid adding extra dependencies, but I can switch to using this library if you're comfortable with it.

I guess it would be useful for future MCP and other AI providers that also uses the same method as well.

[caarlos0]

I've done this "by hand" to avoid adding extra dependencies, but I can switch to using this library if you're comfortable with it.

I think it's probably fine to use it, yes 🙏