tonic: add compression threshold for message encoding

[nossrannug]

Add configurable compression thresholds for Tonic

This PR adds runtime-configurable compression thresholds to improve performance for large message compression scenarios.
Changes:

• Compression threshold: Messages smaller than TONIC_COMPRESSION_THRESHOLD (default 1024 bytes) are sent uncompressed
• Spawn-blocking threshold: Messages larger than TONIC_SPAWN_BLOCKING_THRESHOLD can be compressed on a blocking thread pool to avoid blocking the async runtime

Configuration:
Set via environment variables:

• TONIC_COMPRESSION_THRESHOLD=4096 # Skip compression for messages < 4KB
• TONIC_SPAWN_BLOCKING_THRESHOLD=65536 # Use blocking threads for messages >= 64KB

Benefits:

• Avoids unnecessary compression overhead for small messages
• Prevents large message compression from blocking async tasks
• Maintains backward compatibility (defaults preserve existing behavior)

Implementation notes:

• Added conditional compilation to support builds with/without tokio and compression features
• Compression task polling happens immediately after spawning for optimal waker registration
• Tests updated to verify threshold behavior
• Fixes performance issues with large gRPC message compression blocking the tokio runtime.