RISC-V PVM: expose storage backend in OCaml API

What

The storage backend introduced in !13239 (merged) is exposed in the OCaml PVM library, replacing the dummy implementations in src/lib_riscv/pvm/storage.ml. A dummy PVM state is also introduced.

Why

We need to put in place the OCaml integration that will allow us to develop the PVM entirely in Rust.

How

This change hinges on the correctness of the ocaml-rs-based bindings in src/riscv/lib/src/ocaml_api.rs. The types for Repo, Tree, and Id are exposed abstractly in OCaml.

Manually testing the MR

dune build src/rust_deps
dune build @src/lib_riscv/runtest