Allow running `authgear init` non-interactive

**Problem**
If we setup authgear locally, we need to put the `authgear.yaml` and `authgear.secrets.yaml` into the repo. Problem is putting unecrypted secret files is against security practice. But since `authgear.secrets.yaml` is not optional, meaning whoever wants to setup local environment needs to be able to decrypt the file, which is super unfriendly.

**Solution**
The current version of `authgear init` combines generation of both config and secret file together. To make it both local setup friendly and secure, we need to be able to generate the two files fully automatically, or at least it can generate a workable version if not a full version.

Start by locating the Go entry point for `authgear init` and inspect how it currently generates `authgear.yaml` and the required `authgear.secrets.yaml`. Determine what non-interactive inputs and defaults are available, then verify that local setup can generate both files automatically or produce a workable configuration without exposing encrypted secrets.

Written by the indexing model from the issue text.