[CI] Adding assets.json file to the releases.

[mwu-tow]

Pull Request Description

This PR adds a new file to the release: assets.json, that offers information about the assets available in the release.

The purpose is to have one persistent link https://github.com/enso-org/enso/releases/latest/download/assets.json that has the current download links that can be consumed by the website.

Also, the release template has been updated to use the same assets information source, rather than duplicate the information about artifact names.

Additionally, additional step for release validation was added, so the CI can alert if one of the expected assets is missing.

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.