Add implementation of Storage based on pingora_memory_cache crate to use in HttpProxy

## What is the problem your feature solves, or the need it fulfills?

Being able to use "production-grade" caching out of the box, without needing to dig deep into caching implementation, when only requiring simple in-memory cache.

## Additional context

`pingora::cache::MemCache` has doc-string that says that its `For testing only, not for production use.`.
But it's the only implementer of `pingora::cache::Storage` currently.