Protocol: supports views for enshrined contracts

What

Adds support for views in enshrined contracts.

Why

How

Views are actually related to the implementation. As such, views for native contracts and michelson contracts are now splitted between native and michelson, and only appear in the AST for michelson, while for native contract they are retrieved on the fly.

Manually testing the MR

To make it clearer, the framework and actual implementations of views are in two different MRs. I suggest to test !19980 for that.

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