Allowing Generator to pull the templates from the private GitHub Repository

### Why do we need this improvement?

We have already introduced to pull the templates from the private registries like npm, nexus etc, we can extend it to allow the templates from the github/gitlab repositories as well

### How will this change help?

Developer can use the generator for the internal use case efficiently

### Screenshots

_No response_

### How could it be implemented/designed?

* Add case to handle the templates fetching from the private github/gitlab repositories

### 🚧 Breaking changes

Yes

### 👀 Have you checked for similar open issues?

- [X] I checked and didn't find a similar issue

### 🏢 Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/asyncapi/.github/blob/master/CONTRIBUTING.md)

### Are you willing to work on this issue?
Yes, I am willing to submit a PR

Start by locating the existing code that fetches templates from private npm or Nexus registries and trace how registry authentication and source selection work. Extend that flow for private GitHub and GitLab repositories, then verify that authenticated template retrieval works for the supported repository cases and that existing registry behavior remains intact.

Written by the indexing model from the issue text.