Manager and Transceiver interface

[Oghma]

Summary

Add a soroban-ntt-client crate so Soroban smart contracts can interact with NTT contracts without depending on the full contract WASM crates.

Changes

• Add the new stellar/contracts/soroban-ntt-client crate.
• Expose shared Soroban client bindings and contract-facing interfaces for:
  • NTT manager
  • rate limiting types
  • transceiver
  • Wormhole transceiver
• Move shared NTT types and error definitions into the client crate so external users can depend on a lightweight Rust crate instead of the contract crates themselves.
• Update the manager and transceiver contracts to consume the new shared interfaces/types from soroban-ntt-client.
• Wire the new crate into the Stellar workspace and update Cargo dependencies accordingly.