Avoid using the EntitiesRequest API object in query execution logic code in gateway-service #119

Use Case

This isn't a new use case but a tech-debt. We currently use EntitiesRequest (https://github.com/hypertrace/gateway-service/blob/main/gateway-service-api/src/main/proto/org/hypertrace/gateway/service/v1/entities.proto#L27) to receive the entity query in gateway-service but we use the same API DTO across the implementation code. This restricts us in-terms of how the API or implementation can independently evolve.
This issue is to fix that and introduce an internal object for implementation.
cc @aaron-steinfeld @tim-mwangi