Create a manual database migration trigger

Is your feature request related to a problem? Please describe.

I changed the Pulp resource to use an external PostgreSQL database from the operator-managed instance, which didn't trigger the migration step, so all Pulp services complained about missing migrations which never happened.

Describe the solution you'd like

Create a way to manually trigger the migration job. This could be adding a .spec.migration_job.triggerNonce field, which accepts any random string and runs the job whenever it changes. As an alternative, the migration Job spec could be added to the documentation such that you could manually trigger it with kubectl or similar

Start by reviewing how the operator creates and triggers the Pulp migration Job, then inspect the documented resource configuration and the external PostgreSQL transition described here. The work is complete when users can reliably trigger migrations after changing database configuration, either through a triggerNonce field or a documented kubectl procedure.