Draft: SCORU: WASM: decode eval config from tree

DO NOT MERGE! This MR is all about the last commit - there is a rebase waiting to happen.. I pushed it before going on vacation, so that somebody else can take over.

Context

Decode WASM Eval.config from tree. Missing bits, and things that need fixing:

• Admin instructions.
• Several places use list where we probably want to use Vector.t: in src/lib_webassembly/exec/eval.ml, there is: the stack type, members of the admin instruction and the code type.

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