Create new script to apply Gatheouse DB Schema migrations.

[AshCorr]

What does this change?

Adds a new ./db folder which contains DB schema migrations for the Gatehouse DB and scripts to apply them

The ./migrate.sh script does the following:

1. Finds the correct database details for the specified stage
2. Fetches the admin user credentials. We need admin user credentials here as the migration scripts need to be able to create tables and users.
3. Open an SSH tunnel through our PSQL client instance to the database cluster and makes the DB available locally on port 6543.
4. Run flyway in Docker to list pending migrations.
5. Run flyway in Docker to apply pending migrations.
6. Finally prompts the user if they'd like to rotate the admin user credentials.

Why Flyway?

When looking for a DB schema managed I wanted 2 things:

1. Inbuilt CLI, ideally with a docker image so that developers wouldn't need to install anything.
2. Programatic API, right now these scripts run locally on a developers machine, but it would be nice to eventually apply them from a Lambda instead to improve our security posture.

Flyway seemed to pop up a lot when searching from DB migrator tools and seems quite popular on Github with 20k stars. It has a CLI w/official Docker image which is what this PR is using and it has a programatic API (Java unfortunately, but would work for a Scala lambda)

A runner up I considered was Knex.js which also has a CLI and Programatic API, unfortunately there was no official docker image I could see and it came with a lot of extra functionality that I wanted to avoid for now such as an ORM for JS/Typescript.

How to test

Tested with ./migrate.sh CODE (overrode main branch check)

[github-actions]

Deploy build 370 of identity:gatehouse to CODE

All deployment options

• Deploy build 370 of identity:gatehouse to CODE
• Deploy parts of build 370 to CODE by previewing it first
• What's on CODE right now?

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From guardian/actions-riff-raff.

[coldlink]

Looks good to me! Haven't tried running the scripts myself though, so relying on your testing!