Allow apps to register columns for bigint migrations

Allow apps to register columns for bigint migrations: Similar to the AddMissingColumns and AddMissingIndices commands we should allow apps to register columns that shall be migrated to bigint with the occ db:convert-filecache-bigint command. We'd probably rename that to occ db:convert-to-bigint then to remove the filecache indications, since it does more than that.