[ADP-3185] Export era-related types and functions from Cardano.Write.Era.

[jonathanknowles]

Issue

ADP-3185

[Anviking]

Thanks for tackling, it'd be great to wrap this up and stop importing things from cardano-balance-tx:internal! But I'm confused on the tactics here as I
1] thought we were going for one big (but not that big really) public umbrella module for cardano-balance-tx
2] find the degrees of freedom in

• what public module is RecentEra in
• what internal module is RecentEra in

a bit overwhelming and perhaps overkill.

Although in the end, the place where we really want an umbrella module is the theorized cardano-write Cardano.Write as Write also containing re-exports and helpers for Tx construction...