POC of Github App Github authorisation model

[levgorbunov1]

User Need

As a Developer I want To test if we can replace bot user with github app for authenticating to Github so that We don't need to manage PAT tokens and an insecure bot user.

Description At the moment we use PAT tokens associated with a bot user to authenticate to Github and perform Github operations, this is insecure and unwieldy. This ticket's purpose is to investigate reconfiguring our Github authentication model to use a Github app for authentication.

Acceptance Criteria:

• Create Github App for testing with a given permission set.
• Test authenticating to Github from pipeline.
• Test authenticating through app for api use in application logic.

[AntonyBishop]

Cloud Platform have a similar need. Perhaps when we know the answer we can share our findings - https://github.com/ministryofjustice/cloud-platform/issues/5748