[Feature Request] Manage GitHub Apps via Terraform

Hey Team,

I was looking for a way to manage GitHub apps [1] via Terraform but quickly realized there is none. It would be great to be able to setup a GitHub app, here are some exemplary settings one might want to manage:
- app name
- permissions
- webhook URL and secret
- event subscriptions

Cheers!

### Terraform Version
```
Terraform v0.12.28
```

### References
[1] https://developer.github.com/apps/about-apps/