Compression support

[buinauskas]

Hello, before I begin, thank you for an excellent library!

Recently, in one of our services, we started caching more and have reached maximum available memory, meaning that Redis will evict old keys and our cache-hit ratio suffers.

The async Redis cache stores data using JSON format. It's not the most compact format, and there's little to do with our underlying data structures.

Could compression be supported before caching the underlying data as an opt-in? I am willing to implement it instead of hacking something in our project.

I was planning on using https://crates.io/crates/async-compression and its available encoders.

Thanks!