PParams in cardano-ledger-api

[koslambrou]

As a cardano-ledger-api user, I want it to expose the public API for working with protocol parameters.

We should

• - create the module Cardano.Ledger.Api.PParams
• - expose all datatypes that will allow a user to create and/or work with a PParams for any given era (mostly the EraPParams typeclass)
• - add doctest showing how we can do that

This issue should be done after #2927

[JaredCorduan]

@lehins I think it is fair to say that this issue is resolved by #3328. Do you agree, or did you have plans to add anything else?

[lehins]

add doctest showing how we can do that

Not that part. Let's keep all of the api tickets opened for now and I'll go through them when I can actually focus on the cardano-ledger-api (which should be fairly soon)