feat: add GitHub storage backend support

[pierre-monnet]

This pull request introduces a new "GITHUB" mode to the editor, allowing it to load and save data contract files directly from a GitHub repository. It adds support for configuring GitHub repository details, integrates a secure proxy option to avoid exposing tokens to the browser, and updates both the documentation and the application logic to support these features.

The most important changes are:

GITHUB Mode Support:

• Added a new GITHUB mode to the editor, enabling direct integration with a GitHub repository for loading and saving a single data contract file. The editor can now be configured to auto-load a file from a specified repo and branch, or let the user pick the repo/branch/path at runtime. (CONFIGURATION.md, src/App.jsx) [1] [2] [3]

GitHub Configuration Options:

• Introduced new environment variables and configuration options (GITHUB_OWNER, GITHUB_REPO, GITHUB_BRANCH, GITHUB_PATH, GITHUB_TOKEN, GITHUB_API_BASE_URL, GITHUB_PROXY_URL) to support GitHub integration, and updated the Dockerfile to inject these into config.json. (CONFIGURATION.md, Dockerfile) [1] [2]

Proxy Support for Secure Token Handling:

• Added documentation (GITHUB_PROXY.md) and a reference implementation (github-proxy.js) for an optional server-side proxy that handles all GitHub API requests, keeping the GitHub token out of the browser for improved security in production/multi-user setups. (GITHUB_PROXY.md, github-proxy.js) [1] [2]

Documentation Updates:

• Expanded CONFIGURATION.md with detailed instructions and code samples for configuring and using the new GITHUB mode, including both direct and proxied setups. (CONFIGURATION.md) [1] [2]

App Logic Enhancements:

• Updated the main app logic to automatically load the configured GitHub file on mount in GITHUB mode, and to handle error notifications gracefully if the file cannot be loaded. (src/App.jsx)

[pierre-monnet]

@jochenchrist @simonharrer this PR is ready for review