[MM-58713] Add docker compose and the workflow for integration tests

We basically spin up a MySQL, Postgres and Mattermost instances to emulate real world deployment to run migration-assist against.

The build/docker-compose.yml is to make sure that environment is ready for the tests.
The new "Integration Test" action runs several migraiton-assist commands
- mysql with fix flags
- postgres with mattermost version, downloads the migrations and applies them
- pgloader generates the configuration files
We indeed run pgloader itself with Docker (to cover the guide) to run and test the configuration and migrate the data
The action saves the outputs (eg, .load file and log files for further processing and debugging).

This was the cherry on the top to ensure the tool has a quality control and after this we can go v0.1.

For the future iterations:

We can include plugin migrations
Run some SQL queries to break MySQL database to make use of fixes in the mysql subcommand of migraiton-assist

mattermost / migration-assist