DB schema migration support

[bryanchriswhite]

Background

Currently, have to re-index the entire network any time we want to deploy a schema change. This is undesirable because it increases the complexity of our infrastructure and significantly increases the lower bound on how quickly the indexer can sync up with a given network after a schema change.

Proposal

Consider using migrate which is part of the Graphile suite and is intended to work alongside Postgraphile (which we're also using).

Acceptance criteria

1. Add migrate and dependencies
2. Add a section to the readme explaining or linking to how to use

Test coverage over:
Example use cases:

• adding an entity
• changing a field's data type
• changing a @jsonField type (add/remove/etc. its properties)
• removing an entity (too many questions, need more discussion)

meaning:
1. Alter graphql schema
2. Generate DB schema (a. let SubQuery do its thing b. pg_dump or something to dump schema)
3. Assert agains the DB schema diff

It's sufficient to determine that the tool works as expected.