Provide existing SSM parameters for github_app secrets to avoid storing values in Terraform state

[jo-koe]

We would like to have the option to provide existing SSM Parameters for the github_app.key_base64 and github_app.id inputs so that the values for these secrets are not necessarily stored in the Terraform state. (The input github_app.webhook_secret can be managed via the module soon so we've not mentioned it: #4392).

We've seen that this topic was discussed already several times e.g. #2936, #2183, #587. Also we do understand that this would require some resource management outside of this module which is maybe not wanted. Anyway we just wanted to ask for it again to get a clear answer.

Thanks!

[npalm]

Quick update, the PR is almost ready. Some last minor remarks need to processed.