V3 Subgraph: Define entities to be indexed by the subgraph.

[kkennis]

Issue #41 will enumerate the events that can are relevant to our data model - this task should actually do the work of defining that data model. This will require looking at prior art - the Uniswap V2 Subgraph, and defining the tables to be stored in our database.

Things to think about:

• At least data parity with V2 - may need additional columns to represent e.g. discrete liquidity concepts
• Possible time intervals - V2 supported daily and hourly. We may want to index intervals as tight at 1 minute.
• Should make sure that all entity columns are either directly reflected by or can be derived from the payload of our relevant events.

Acceptance Criteria:

• File schema.graphql defined in PeggyJV/uniswap-v3-subgraph representing the defined entities

[jackzampolin]

Closing as complete