As a step towards #151

, we can introduce the start of our GraphQL API.

We can start by adding support for DMD's inbuilt metadata table, which
right now has just the version of the CLI that was used to bootstrap the
database.

This introduces a separate CLI, `dmd-graph` that can be used to serve
the GraphQL API, as well as wiring it into the `dmd-web` CLI.

Co-authored-by: Keith Lyall <keith.lyall@deliveroo.co.uk>

Closes #320.