No Cargo workspace configuration — three contracts each have standalone, duplicated Cargo.toml files

[abayomicornelius]

fee_collector/Cargo.toml, stellar_send/Cargo.toml, and token_bridge/Cargo.toml are nearly identical (same soroban-sdk = \"21.0.0\" dependency, same [dev-dependencies] and [features] blocks) but there is no root-level Cargo.toml defining a [workspace], even though the README's "Clone and install" section explicitly states "The workspace Cargo.toml pins all dependency versions."

This means cargo build/cargo test cannot be run once from the repo root, dependency versions can drift between contracts over time, and there's no single Cargo.lock for reproducible builds. Add a workspace root Cargo.toml with members = [\"fee_collector\", \"stellar_send\", \"token_bridge\"] and hoist the shared dependency versions.