support computed keys for data fetching composables

[danielroe]

There are a number of cases where users want to pull data in (often based on route parameters) and dynamically update this data as things change. (For example, #14581, #14342, #14493.)

It would be useful to add support for computed keys in useFetch and useAsyncData so that the original data fetched isn't overwritten but rather a new key/asyncdata value is accessed based on the computed key. This would also have the effect of meaning that initialCache would not have to be set to false to enable updates on route navigation.

[pi0]

One major issue with supporting computed keys, is that we might endup with (mostly client-side) memory leaks when input dynamically changes. There might be better ways to overcome with limitations of dynamic route keys (like using a subkey). Let's wait for it until fixing asyncData shared state.