feat: add support for assets in private repositories

[hhromic]

This PR adds support for downloading assets from private repositories in eget.

I had to do some code improvements to facilitate this feature, which I implemented in separate commits for an easier review:

• chore(http): send token using standard Bearer authorization scheme
  • the Bearer scheme also allows to use JSON web tokens (JWT)
  • ref: https://docs.github.com/en/rest/overview/authenticating-to-the-rest-api
• chore(http): allow to provide Accept header in Get()
  • all binary downloads should use the application/octet-stream media type
  • all GitHub API calls should use the application/vnd.github+json media type
  • also refactor GetRateLimit() to use Get() instead of own HTTP request
  • ref: https://docs.github.com/en/rest/overview/media-types

The main changes to support assets in private repositories are in the third commit:

• feat: add support for assets in private repositories
  • download assets through the GitHub API instead of browser download links
  • allows to download from private repositories when combined with a GitHub token
  • added new Asset type with the Name (necessary for matching) and DownloadURL of an asset
  • ref: https://docs.github.com/en/rest/releases/assets

The key aspect is using url instead of browser_download_url in the assets JSON response from GitHub, which always works for both public and private repositories (when using a GitHub token). However, the asset URL uses asset IDs instead of asset filenames. Therefore, I also introduced an Asset type that contains both, the Name of the asset and the DownloadURL.

Then, the majority of the changes are refactoring functions to use Asset instances instead of a plain string. In this way, all functions remain with access to the actual asset name regardless of the URL and thus private assets work seamlessly.

I tested this with both public and private repository assets, and with and without using a GitHub token.
I also ran make test and all the defined tests pass OK.

[rinshadka]

Hi , any update on this feature to be merged soon :)