Introduce DatabaseMigration API and custom resource definition

Summary

DatabaseMigration API provides the interface for runtime specification and workload definition for running GitLab Rails database migrations. The interface MUST:

• Match the capabilities and requirements of ApplicationServer and JobProcessor.
• Support defining database migrations required for upgrading GitLab Rails application, including pre and post deployment migrations.

Success criteria

• API types are defined in api/v2alpha2 package and they support any database migration that is required for upgrading GitLab Rails application.
• CRD can be generated from API types.
• A sample custom resource is provided in config/samples.