Shared library for client requests and payload construction.

[Avi-D-coder]

The CLI and any future SDKs should use a Rust library to identify L2 endpoints and construct/sign payloads.
The library should be WASM compatible for use in TypeScript and Python SDKs.

This logic needs live in Rust to allow the use of efficient formats like rkyv in the signed payload that needs to be checked in ZK.