aqua.yaml's registries: support private field for private registries

[suzuki-shunsuke]

Feature Overview

Similar to #1466 , but for not private packages but private registries.

Why is the feature needed?

Please explain the problem you want to solve.

Same as #1466

Does the feature include Breaking Changes?

No.

Example Code

command and configuration

$ 

https://aquaproj.github.io/docs/reference/config#github_content-registry

registries:
- name: foo
  type: github_content
  repo_owner: aquaproj
  repo_name: aqua-registry
  ref: v3.90.0 # renovate: depName=aquaproj/aqua-registry
  private: true
  path: registry.yaml