feat: make gRPC support optional via "grpc" feature

[zond]

Summary

• Adds a grpc feature to make tonic/prost dependencies optional
• Enables building reticulum-rs for embedded platforms without protoc

Problem

The Kaonic interface requires tonic/prost for gRPC communication. These dependencies:

• Are heavy (increase compile time and binary size)
• Require protoc at build time
• Are problematic for embedded targets like ESP32

Solution

• Add a grpc feature (enabled by default for backward compatibility)
• Make tonic, prost, and tonic-build optional dependencies
• Gate the kaonic module on the grpc feature
• Gate kaonic examples with required-features

Usage

# Default build (includes gRPC, same as before)
cargo build

# Build without gRPC (no protoc required, smaller binary)
cargo build --no-default-features --features alloc

Testing

• All 13 tests pass with default features
• All 13 tests pass without grpc feature
• Successfully builds for ESP32-S3 without grpc

🤖 Generated with Claude Code

[nibytes]

So wait, thank you!