Create request caching service

[michaelneale]

This request caching service will be used to:

make requests to url
return cached data for url
request new data from backend at url
update cache for url
callers of this will get two responses, like a stream of data where the first response is the cached response and the second response is the network response

perhaps using this as a custom hook similar to https://tanstack.com/query/latest (but for dart of course and in the flutter code)