RPM Support

In this previous issue [0], support was added for parsing the "vendor" field on RPM metadata, so that it could be used with search.

This will not affect RPM metadata already in the database, only new RPMs, so in order to get consistent results when searching by the vendor field, we need a migration to update this.

[0] https://pulp.plan.io/issues/2781

It is possible that this migration will take a long time to run. This migration should be tested with a large dataset to determine if it would be better to run it outside our normal migration workflow.