Support Generic Webhook Calls to GitHub with GitHub App Authentication

I'm not sure if this issue should be submitted in this repo or in [argocd-notifications](https://github.com/argoproj-labs/argocd-notifications), hopefully here is ok.

Currently the [github service](https://github.com/argoproj/notifications-engine/blob/master/pkg/services/github.go) supports only the [Create Status](https://github.com/argoproj/notifications-engine/blob/1a5452c599569487c6422b3a3427a34ab16e4116/pkg/services/github.go#L190) api.

Notifications to other GitHub API endpoints can be achieved using the generic [webhook service](https://github.com/argoproj/notifications-engine/blob/master/pkg/services/webhook.go), but this method does not support [GitHub App Authentication](https://github.com/argoproj/notifications-engine/blob/1a5452c599569487c6422b3a3427a34ab16e4116/pkg/services/github.go#L132).

GitHub App authentication is preferable to me because at my organization it can be quite difficult to obtain a GitHub machine/service user account, but it is quite easy to receive a new GitHub App instance.

Please provide the ability in the [GitHub service](https://github.com/argoproj/notifications-engine/blob/master/pkg/services/github.go) to build custom notifications to GitHub. I believe the majority, if not all, of the [GitHub REST API](https://docs.github.com/en/rest/reference) could be supported with the current implementation of the [webhook service](https://github.com/argoproj/notifications-engine/blob/master/pkg/services/webhook.go), so a similar (or exact duplicate) implementation but capable of authenticating using a GitHub App would be greatly appreciated.

There are 2 possible implementation options that come to mind:

1. Add a new method in the GitHub service that is capable of doing custom webhook invocations
2. Add a GitHub App authentication method to the generic webhook service