Provide support for github apps

[blixem777]

Preflight Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

From the docs, it seems there is support for github oauth2 apps, but not for github apps.

Since github is recommending using github aps instead, it would be great if there was support for that in dex.

Proposed Solution

Something like the below:

connectors:
- type: github
  id: github
  name: GitHub
  config:
    appID: YOUR_APP_ID_FROM_GITHUB
    appPrivateKeyPath: /path/to/your/private-key.pem
    ...

Alternatives Considered

No response

Additional Information

No response

[nabokihms]

@blixem777
Is this more secure than using oauth2 flow for authentication? What is appPrivateKeyPath?