Option to cache resolved stylesheets

[Stranger6667]

Generally, customizable resolvers will be nice (eg to provide a way to use the fetch API in WASM instead of our default impl). When implemented, we can ship a caching resolver that will be configurable in terms of the cache size / ttl.

The use case is when you have many emails to inline and all of them use the same remote stylesheet - caching would be of great help there.