chore(sdk): Adds Rust example for message compression using user_headers

[haubur]

Rust example: Per message compression using user_headers

Closes #30

• Consolidates exisiting example message-headers into message-headers/message-type
• Adds new example to message-headers/message-compression
• Adds a producing example message-headers/message-compression/producer/main.rs
• Adds a corresponding consuming example message-headers/message-compression/consumer/main.rs
• Adds codec.rs to examples/rust/src/shared that implements shared utilities to compress and decompress messages along with methods to transparently resolve user_headers into a codec.
• Adds README.md with some more details, to keep code examples readable.