PParams in cardano-ledger-api

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

We should

* [x] - create the module `Cardano.Ledger.Api.PParams`
* [x] - 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 https://github.com/input-output-hk/cardano-ledger/issues/2927