Enhance store for querying multiple entities

Do you want to request a feature or report a bug?

Feature

What is the current behavior?

Currently the host store interface only supports interacting with one entity at a time:

/**
 * Host store interface.
 */
export declare namespace store {
  function get(entity: string, id: string): Entity | null
  function set(entity: string, id: string, data: Entity): void
  function remove(entity: string, id: string): void
}

What is the expected behavior?

The store should expose functions similar to the GraphQL interface like:

let entities = Entity.findAll(where: { blockHash: "0xdeadbeef" });

Or even expose the SQL interface?

Start with the host store interface shown in the issue and compare its single-entity get, set, and remove operations with the Entity.findAll GraphQL example. Clarify whether multi-entity querying or SQL exposure is intended; done requires an agreed query interface and defined behavior.