[Tooling] Add graphile-migrate / plv8 migration support

[bryanchriswhite]

Objective

Support explicit and version controlled DB migrations via graphile-migrate and plv8/plv8ify.

Origin Document

DB migrations are implicit and automatic by default in the subquery framework. This introduces risk in the production environment and implies a necessity to re-index post-migration, which would otherwise be unnecessary due to the forward compatible nature of the current design (i.e. primitives).

Goals

• Incorporate schema & data migrations into version control to improve reproducibility and transparency.
• Add support for data migrations, leveraging the forward compatible design of the primitive entities.
• Eliminate the need to re-index after schema changes & support typescript source migrations.

Deliverables

• Restore the "DB Migrations" section of the README (from fetchai/ledger-subquery)
• Replace the postgresql DB image in the docker-compose.yml with a version which includes the plv8 extension.
• Add package.json scripts and placeholder files/directories in order to support plv8(ify)/graphile-migrate workflow (see: fetchai/ledger-subquery package.json db:* / plv8ify)

Non-goals / Non-deliverables

• Adding any migrations.

General deliverables

• Comments: Add/update TODOs and comments alongside the source code so it is easier to follow.
• Testing: Add new tests (unit and/or E2E) to the test suite.
• Makefile: Add new targets to the Makefile to make the new functionality easier to use.
• Documentation: Update architectural or development READMEs; use mermaid diagrams where appropriate.

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Creator: @bryanchriswhite