RISC-V/PROTO: Update riscv backend types

Closes of RV-490

Part of RV-374

Blocked by RV-443 & RV-407 (SDK & PVM not handling metadata reveals through the raw reveal mechanism.)

What

Adds required types for protocol integration to octez-riscv-pvm's backend module. They need to correspond with the ones introduced in !16163 (merged). You can check everything types together properly in !16752 (closed)

Why

Unblocks changing the mocked protocol environment module Riscv_proto_env to the actual riscv backend exposed by the protocol enviornment through octez-riscv-pvm OCaml module. In particular this change will enable type equalities all the way from the protocol through backend.ml

How

The types in interface backend.mli should correspond with the types used in sc_rollup_riscv.ml - the protocol implementation. These types are introduced in this MR: !16284 (merged)

Manually testing the MR

make -C src/riscv build
make
dune exec tezt/tests/main.exe -- riscv

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

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
  • make -C src/riscv
  • dune build src/lib_riscv
  • dune build src/rust_deps
• Benchmark performance and populate the table above if needed.
• Explain changes to regression test captures when applicable.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.