Env/Plonk: abstract public_parameters type

Context

Plonk's verifier expects as arguments a transcript and some verifier_public_parameters. Both these values are part of the output of Plonk's setup phase, and they are fixed after that step. In the interface exposed in the environment, they are always used together and never modified. This MR merges these two types into a new abstract type in Plonk's environment interface.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR