Implement backend support for pulling APIs from public Git repositories

Description

This sub-issue focuses on implementing the backend REST API required to fetch the contents of a public Git repository as part of enabling API import from remote repositories.

Currently, only public repositories will be supported.
Support for private repositories (with authentication) will be added later.

Scope

POST /git/repo/fetch-branches

Purpose:
Fetch the branches of a given public GitHub repository to verify accessibility and display its branches and their status (isDefault) in the API Platform UI.

Request Payload:

{
    "repoUrl": "https://github.com/example-org/sample-api",
    "provider": "github"
}

POST /git/repo/branch/fetch-content

Purpose:
Fetch the contents (directories and files) of a given public GitHub repository to verify accessibility and display its structure in the API Platform UI.

Request Payload:

{
    "repoUrl": "https://github.com/example-org/sample-api",
    "provider": "github",
    "branch": "main"
}

Behavior:

Fetches and returns the directory and file structure of the specified repository and branch.

Fails gracefully if:

• The repository is private or inaccessible.
• The repository URL is invalid or not a valid Git URL.

Error Handling

Return a descriptive error if the repository cannot be accessed (e.g., private or invalid URL).
Handle network or Git fetch errors cleanly with appropriate status codes.