Expand TxAuxData interface

[lehins]

Description

Because TxAuxData isn't really used in the ledger rules it has been neglected whenever we did the refactor of the ledger interface using lenses.

Now that we expanding our testing and have a need for era agnostic type construction we could use the same interface for TxAuxData as for other type families in ledger.

This PR introduces:

• Cardano.Ledger.Metadata module with Metadatum type moved to it from cardano-ledger-shelley
• mkBasicTxAuxData and metadataTxAuxDataL to EraTxAuxData type class.
• AllegraEraTxAuxData type class with timelockScriptsTxAuxDataL lens
• AlonzoEraTxAuxData type class with plutusScriptsTxAuxDataL lens

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

[teodanciu]

Great! (and almost definitely what I should have done in my other PR instead of assuming there is some reason to not have it and hacking my way around it).
Looks good to me, so nice to have this loose end tied up.