Execute DDL - Ecto.Adapters.SQL.Connection

Part of #38

https://hexdocs.pm/ecto_sql/Ecto.Adapters.SQL.Connection.html#c:execute_ddl/1

This is the function we use to run migrations. The existing implementation of this has multiple function clauses, so it's likely that we can use some of them, and override others.

It's here that we can ensure certain columns are added to tables when they're created, as well as defining the cid as the primary key.