Introduce a general caching abstraction to wasix

[Michael-F-Bryan]

Motivation

There are many places across the Wasmer CLI and WASIX where we want to use caching to avoid unnecessary work. At the moment, each of these places is hand-rolling its own caching solution.

Off the top of my head, I can think of the following:

• wasmer_wasix::runtime::resolver::WapmSource - caches the response of a GetPackage() query against the registry
• wasmer_wasix::runtime::module_cache - contains various implementations of caches for WebAssembly modules (in-memory, thread-local, filesystem, etc.)
• wasmer_wasix::runtime::package_loader::BuiltinPackageLoader - caches *.webc files downloaded from the registry
• wasmer_wasix::runtime::resolver::WebSource - caches *.webc files downloaded from the internet via a bare URL

These caching solutions all tend to have the same properties or requirements:

• It's a key-value store where the keys are arbitrary strings
• We want the files to be cached on disk so they can be picked up from subsequent wasmer runs
• The values are often "big" and we want to mmap (via shared_buffers::OwnedBuffer) them rather than reading into memory
• On-disk caches need to be first saved to a temporary file and moved into place to avoid seeing results before they've finished being written
• Cached values should be kept in memory once loaded from the filesystem
• We often want a way to invalidate a cache key, whether that is via a timeout, by checking if an ETag header or hash has changed, or whatever
• We want the option to use stale values if the "main" method of fetching the data fails (e.g. because of an network error)
• We need to work in both sync and async contexts

Proposed solution

I was thinking of creating a concrete type with an API similar to a HashMap<CollectionName, HashMap<Key, Value>>, except it'll pass out instances of shared_buffer::OwnedBuffer and automatically manage the synchronisation of on-disk and in-memory caches.

This would probably hook into Wasmer Edge's caching facilities, too.

Additional context

This originally came up when I was working on #3983. Having one or two places where we do caching is fine, but I noticed I was doing the same in-memory/on-disk dance in several places and all of it is essentially untested.

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