Enable RustlerPrecompiled to Fetch NIFs from Exstatic (private repo)

[Primebrook]

Summary

Building on #4, which introduced RustlerPrecompiled for fetching precompiled binaries, this PR adds the Exstatic.ReleaseHelper module to enable authenticated downloads from the Exstatic repo. Since Exstatic is private, direct public URLs are not an option. Instead, this module implements GitHub’s private asset download flow, for retrieving the NIF binaries.

I also added an up to date CHANGELOG.md in this PR.

Challenges with using RustlerPrecompiled for private GitHub releases

As discussed in philss/rustler_precompiled#65 (comment), fetching assets from private repositories requires an extra couple of steps because:

• Direct downloads are not allowed (unlike public repos).
• The download URL is dynamic – assets are stored in GitHub’s S3 backend, requiring a separate API request to retrieve a signed URL.
• Requests must be authenticated – all API calls must include a GitHub Personal Access Token (PAT). For this, users must create and set the PAT in the env varEXSTATIC_GITHUB_TOKEN (I'll add this to the README in a follow up).