feat: store/v2 should not require eager mounting of all store keys

[kocubinski]

Previously in store/v1, and now in store/v2, a list of all potential store keys (now called actors) must be provided at initialization of store in the application lifecycle. See the handling of opts.StoreKeys in store/v2/root/factory.go. In reality this is an implementation detail of IAVL and nothing else, in IAVL one store key corresponds to one state root.

We propose that store/v2 should not require all possible actors to be defined at construction, and should support the creation of new actors at runtime. In order to support this, a small piece of code should wrap IAVL KV operations to provide special handling of keys which are prefixed with an eagerly supplied (only the case of IAVL) list of prefixes. @testinginprod drafted a poc of an approach for this previously, along with some benchmarks and ideas for prefix -> byte slice matching.