Reverse lookup readside handler

[AntonioYuen]

Is your feature request related to a problem? Please describe.
It is very common to create readsides that are purely reverse lookups. Given, the event and state returns a string, we should be able to automate it.

Describe the solution you'd like

1. RPC called CreateReverseLookup(event, state, meta) and returns List((K, V)) pairs where Key is the field_name.
2. Akka projection that runs CreateReverseLookup
3. RPC for searching for entityId by reverse lookup (K, V) pair.

Maybe we will make a solution for multi-key lookups.

Describe alternatives you've considered
N/A

Additional context
It is a common use case to want to offer reverse lookups for entities.