Pre-generate `github-webhook-secret`

### TL;DR

When instantiating the module, v1 of `github-webhook-secret` is created. Then, README asks to generate this secret locally (32 char base64) and update the value in the cloud console (creating v2).

Since this value is not inserted anywhere else, I don't understand why Terraform can not create the "final" value directly as v1, removing the need for this manual step? I think it should be able to create an appropriate string with `random_password`.

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

Doing click-ops to update the secret values in Cloud Console is not great as it is not captured by Terraform.

This is also the case for the .pem secret, which presumably could also be imported into terraform, but I understand if this is a little trickier.