[Rel-Eng]: Adds script to automatically generate GHCup metadata for Cabal releases (#9411)

* [Rel-Eng]: Add initial version of the release metadata script

Adds the initial version of the release script that generates
the metadata for a given release automatically in the correct format
which can be used by GHCup as-is which makes it way easier to add
that release to be distributed via GHCup.

This would evolve based on the feedback from the maintainers but
the basic core functionality still works.

To Resolve: #9298

* [Rel-Eng]: Change top shebang from '/bin/bash' to '/usr/bin/env bash'

Modifies the top shebang from being #!/bin/bash to #!/usr/bin/env bash
as requested in the PR review

* [Rel-Eng]: Add script to download cabal-install binaries for a release

The script to generate GHCup metadata needs the artifacts already downloaded.
This script was made to be used in conjunction to the ghcup metadata generating
script.

This script:-

* Makes the directories that ./create-release-metadata-for-ghcup.sh CDs into
* Downloads the binary release files given a particular release number into
  correct directories.
* Verifies that the downloaded checksums match the expected values upstream.

* [Rel-Eng]: Integrate the release binary downloading script into GHCup metadata generating script

Checks if binary-release folders for the release passed exist.
"create-release-metadata-for-ghcup.sh" needs the release files pre-downloaded.

Echos user to run the download script first if correct binaries folder not
found.

* [Rel-Eng]: Add release-scripts binary-downloads to gitignore

We do not want to commit the downloaded binary files for a cabal release.
This prevents us from doing that, even on accident.

* [Rel-Eng]: Change from '/bin/bash' to '/usr/bin/env bash' in the download script

Modifies the top shebang from being #!/bin/bash to #!/usr/bin/env bash
in the 'download-cabal-install-release-binaries.sh'

The later seems more portable as learned from the PR review comments.