custom CA for github endpoint should be configurable via a secret Reference

[bavarianbidi]

What is the feature you would like to have?

It's possible to create a github endpoint with a custom CA.
Previously (garm <= 0.1.4) it was part of the configuration file where it was possible to point to a CA file on the server.

The new endpoint API in garm allows the create/update with a byte slice.
We should allow the same by referring to an existing kubernetes secret.

Anything else you would like to add?

No response