Implements the encoding level architecture suggested by @kentonv in #133, handling serialization at the transport layer rather than through a global WireFormat hook.

Summary

• Add EncodingLevel type with four levels: stringify, devalue, partial, passthrough
• Add optional encodingLevel property to RpcTransport interface
• Add wrapTransport() helper for custom binary formats (CBOR, MessagePack, etc.)
• Serialize/deserialize based on transport's encoding level
• Maintain full backwards compatibility (stringify is the default)

Encoding Levels

Motivation

Binary formats like CBOR can significantly outperform JSON for large payloads by avoiding base64 encoding cycles. This architecture lets transports declare their capabilities and have serialization handled appropriately.