Include metadata in the release descriptions for diagnostics

[agriyakhetarpal]

Currently, we release two release cross-build environments as artifacts: one with Pyodide built in debug mode, and another in release mode. However, it is not convenient for users to know how to use the xbuildenvs.

I think we should provide diagnostic information about the xbuildenvs. This can include information such as:

• The Emscripten version being used for Pyodide – this helps users know which version of Emscripten they need to activate/download/install for building packages
• Package versions – it would be helpful for users to know the versions of cross-build-env packages (both static/shared libraries as well as Python packages) at a quick glance.
• Build metadata – size of the Pyodide runtime, and a tree of the files included
• Checksums – SHA-256 checksums of the files included (similar to RECORD files inside .dist-info folders.
• Licenses? – I'm not sure how we would go about doing this

This can be provided as a set of Markdown tables (with a template that we can fill out using a Python script, or just through shell commands), and as a JSON file in the release artifacts (which will be machine-readable, if anyone ever needs such information).

This is slightly related to SBOMs, but not quite the same, as they have a specific format we will need to comply with.

[ryanking13]

Sounds good to me.