Add ClickHouse structure SQL and rake tasks

What does this MR do and why?

• Adds tasks for loading schemas, as well as dumping schemas during migration and rollback
• Adds documentation on how to load schema
• Updates test environment to load from schema, not run migrations.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Run ClickHouse migrations, and assert that a file db/click_house/main.sql has been created with the schema dump.
• Run tests that are annotated with the RSpec :click_house type. These tests should all run as before, although the tables are generated using the schema dump and not the migrations.
• DROP all tables in the development database (if you can), and run bundle exec rake gitlab:clickhouse:load_schema:main to repopulate the schema.

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