Introducing projections

[huntc]

Here are the traits and types being proposed. These correspond reasonably well with their JVM counterparts but with the queries conflated into the providers as discussed offline:

Along the way, I've also dropped the naming of Record for entities and now use EventEnvelope, which is consistent with its use elsewhere.

TODO (in another PR):

• implement the source provider for the commit log
• implement the projection runner
• implement the gRPC source provider

[patriknw]

looks like a good starting point that can be merged (soon)

[patriknw]

Might be missing the slice range that this projection should handle, but slices can come later in a separate step and we start with all slices.

Corresponding jvm example https://doc.akka.io/docs/akka-projection/current/r2dbc.html#running-with-sharded-daemon-process
or with grpc https://doc.akka.io/docs/akka-projection/current/grpc.html#consumer

[patriknw]

btw, here is the jvm code for slice ranges https://github.com/akka/akka/blob/8011e8e2de56502771336140a1c1dd84d3b97de6/akka-persistence/src/main/scala/akka/persistence/Persistence.scala#L446-L478

[huntc]

TBH, I've not put a ton of consideration into the slice mechanism just yet. I'll tackle that with the next PR as you suggest.

[patriknw]

LGTM