VideoPress: add logic to migrate metadata fields used on filtering

We recently added new filters for the videos based on some metadata fields #26777. We also started to save this new metadata fields when a change is made on a given video #26769.

Now we need to migrate/create this new fields for all the VideoPress existing videos on a given site, so the filtering for existing videos work as expected.

Some ideas:

* Do this as a background process the first time the VideoPress dashboard is connected
* Let the user know in the UI that there is some migration to be done and ask for an action on it

Start by reviewing the changes described in issues #26777 and #26769, then trace the VideoPress dashboard connection and existing-video metadata handling. Define whether the migration is triggered in the background or from the UI, and consider the work done when all existing videos have the new fields and filtering works for them.

Written by the indexing model from the issue text.