Fix rustapi-grpc documentation for direct usage and add explicit Tokio features

[Copilot]

Addresses review feedback from PR #118: rustapi-grpc examples imported from rustapi_rs::grpc, making them unusable for direct crate consumers, and lacked explicit Tokio feature declarations.

Documentation updates:

• README now distinguishes via-facade vs. direct usage upfront
• All examples updated to use rustapi_grpc + rustapi_core imports (copy-pasteable for direct users)
• Crate-level docs lead with direct usage pattern, mention facade as alternative
• Function doc examples (run_rustapi_and_grpc_with_shutdown) also updated

Dependency improvements:

• Added explicit Tokio features: rt, sync, macros (follows pattern in rustapi-core, rustapi-extras, rustapi-ws)
• No longer relies on implicit Tokio defaults or workspace configuration

Before (confusing for direct users):

use rustapi_rs::grpc::{run_rustapi_and_grpc, tonic};
use rustapi_rs::prelude::*;

After (works for both direct and facade users):

// Direct usage
use rustapi_grpc::{run_rustapi_and_grpc, tonic};
use rustapi_core::{get, RustApi};

// Or via facade (mentioned in docs)
use rustapi_rs::grpc::{run_rustapi_and_grpc, tonic};

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