storage: generate schema and queries automatically

[yliang412]

Description

The current user experience of integrating new operators with persistent storage is not great. To add a new operator, we need to define the schema of the operator table and edit each memo table API that relies on operator-specific implementation.

Luckily, the operator schema and the queries we need to run exhibit a simple structure that could be integrated into the codegen pipeline.

Requirement

We do not have everything generated in a single pass. The following tasks can be gradually introduced into the system.

• Generate the insertion queries based on the rust operator struct definition
• Generate the union all queries for getting all expressions in a group.
• Write a CLI tool that integrates with sqlx-cli to generate the migrations
  • operator schema
  • indexes
  • maybe triggers, TBD

Originally posted by @skyzh in #26 (comment)