Support database pre-migrations

What does this MR do?

Pre-migration is required for near-zero upgrades.

• MigrationTask is renamed to RunMigration
• change the initialRootPassword type to LocalObjectReference, as it is being mounted directly into the migration job.
• add PreMigration parameter to the migration task
• a unit test is added
• minor improvements for migration annotations

Author's Checklist

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Ensure a release milestone is set.
• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline on GitLab.com.
• When ready for review, MR is labeled workflowready for review per the MR workflow.

Expected

• Test plan indicating conditions for success has been posted and passes.
• Documentation is created or updated.
• Tests are added.

Related issues

Closes #44 (closed)