BIGINT migrations for apps

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Is your feature request related to a problem? Please describe.
As a Nextcloud app developer I might run into the trouble of too big values for integer columns. Server had this issue too, so columns were migrated to BIGINT. The migration is slow and happens in a dedicated command occ db:convert-filecache-bigint. That command have a hard-coded list of tables and columns to convert. Apps have no way of using this mechanism.

Describe the solution you'd like

1. Offer an API for apps to register columns to migrate.
2. Have a unified setup check.
3. Have a unified command for migration.

This can be identical to the mechanism we have to optional indices.

Describe alternatives you've considered

N/a

Additional context

N/a

Start with the existing occ db:convert-filecache-bigint command and the optional-indices mechanism mentioned in the issue. Trace how tables and columns are registered and how setup checks and migrations are exposed. Done means apps can register BIGINT conversions and use a unified setup check and migration command.