CLI era command groups

cardano-cli is transitioning to a new command structure where instead of each era-senstivie command getting an era flag (eg --babbage-era, --conway-era), we will have the era as a top-level command.

For the moment, this will be:

• byron: already exists and will be unchanged
• shelley: re-purposed to mean shelley-era only rather than shelley-based era
• allegra: allegra era only
• mary: mary era only
• alonzo: alonzo era only
• babbage: babbage era only
• conway: conway era only

Then we will have the following:

• latest: the latest supported era. The latest officially supported era in the CLI
• experimental: the era following the latest era that is not yet officially supported in the CLI
• legacy: the commands that existed before the restructuring. Gradually these commands will be migrated to fit into the new command structure.

The latest command group is implied, so any sub-commands there-in will also be available at the top-level for convenience.

The legacy command group is also implied, so any sub-commands there-in will be available at the top-level for convenience provided that the latest version of that command does not yet exist.