test: add HTTP basic authentication to git test server

[knqyf263]

Description

Add HTTP basic authentication functionality to the Git test server in gittest package. This allows testing authentication scenarios in Git repository scanning.

Key Changes

• Added Options struct with Username and Password fields to configure authentication
• Modified NewServer and NewTestServer to accept Options parameter directly
• Implemented authentication using gitkit's AuthFunc
• Added tests for both environment variable and URL-embedded authentication

Test Coverage

The implementation includes tests for:

• Environment variable authentication (GITHUB_TOKEN, GITLAB_TOKEN)
• URL-embedded credentials (username:password@host format)
• Authentication failure scenarios
• Local repository scanning after authenticated clone

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).