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

[Object905]

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.