`rust-client` marked as no_std but unconditionally depends on tonic (breaks no_std builds)

[rust-client](https://github.com/0xMiden/rust-sdk/tree/next/crates/rust-client) is marked as `no_std`. It uses [tonic](https://github.com/grpc/grpc-rust/tree/master/tonic) as a non optional dependency. Since tonic is not no_std, `rust-client` cannot be compiled for bare metal targets.

Examine the Cargo.toml in crates/rust-client to see the unconditional tonic dependency and the no_std declaration. Check if there are any feature flags or conditional compilation attributes that could make tonic optional. Look for existing no_std builds or tests to understand the expected compilation targets. The fix likely involves making tonic a conditional dependency or removing the no_std claim.

Scritto dal modello di indicizzazione a partire dal testo della issue.