Add support for self-hosted SCM

[hunter]

Description

This PR adds support for optionally specifying a SCM to support self-hosted environments. The change defaults to github for backwards compatibility but should support Gitlab or GitHub self-hosted/enterprise deployments on a custom domain.

Before

chain-bench scan --repository-url <REPOSITORY_URL> --access-token <TOKEN> -o <OUTPUT_PATH>

After

chain-bench scan --repository-url <REPOSITORY_URL> --access-token <TOKEN> --scm-platform gitlab -o <OUTPUT_PATH>

Without SCM, chain-bench will fallback to Github SaaS - chain-bench scan --repository-url <REPOSITORY_URL> --access-token <TOKEN> -o <OUTPUT_PATH>

Currently only tested on Gitlab but opening the PR early for discussion. The PR also includes a small fix for Gitlab querying protected branches where repo names don't match the escaped path.

Related issues

• Closes #110

Checklist

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

[CLAassistant]

All committers have signed the CLA.