gRPC streaming read side interface

[zenyui]

Is your feature request related to a problem? Please describe.
it would be cool to offer a streaming gRPC interface for the read handler

Describe the solution you'd like
add a streaming read handler service to the protos, then make read handlers configurable to use the streaming version instead of the unary version

Describe alternatives you've considered
n/a

Additional context
eventually, we could implement projections as streams internally and get even lower latency, etc.