[RFC-007] Add command to generate GitHub App secret

[stefanprodan]

To improve the UX of the GitHub App integration, we should add a command to the Flux CLI so users can easily generate the Kubernetes Secret used by source-controller and image-automation-controller.

flux create secret githubapp my-gh-app \
--app-id=<string value> \
--app-installation-id=<string value> \
--app-private-key=<local path to pem file> \
--app-base-url=<GitHub Enterprise API URL>

The flags should map to the Secret.stringData keys specified by: https://github.com/fluxcd/pkg/blob/3431579a0c60b44e75ac8434030e36d5192070fb/auth/github/client.go#L31-L36

Refs:

• #4846
• fluxcd/source-controller#1647
• fluxcd/image-automation-controller#780