In Drupal the default field storage is in an SQL database. I am creating a custom field type and want it to have non-SQL storage.
How does one do such a thing in Drupal 8?
To give an example usage, I am interested in implementing something similar to Drupal 7's hook_field_storage_load() so I can have fields that load their data from files in the file system.
If it is too complex a thing to give a code example for, a point in the direction of the relevant API and/or default SQL implementation that I can reference should suffice.