Added cardano-ledger-conformance

[Soupstraw]

Description

Adds the cardano-ledger-conformance package, which imports the executable spec generated from Agda. Currently it pulls the generated code from Github, but hopefully we can do that via Nix in the future without the need for an additional repo.

Resolves steps 0 and 1 in #3811.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

Nice!

[teodanciu]

Looks good to me