Cache programmatic API - support asynchronous value loader

[mkouba]

Description

The value is currently loaded synchronously, i.e. there is Cache.get(K, Function<K, V>). It might be useful to add an asynchronous variant so that the value can be loaded by async APIs (without blocking).

Implementation ideas

We could add something like Cache.get(K, Function<K, Uni<V>>).

[quarkus-bot]

/cc @gwenneg(cache)

[mkouba]

Note that due to type erasure we would have to use a different method name (?) or introduce a new interface such as:

interface AsyncValueLoader<K,V> {
   Uni<V> load(K key);
}

and then the new method could be something like io.quarkus.cache.Cache.get(K, AsyncValueLoader<K, V>).

[mkouba]

Actually, the new interface would not help in case of lambda arguments :-(