Allow setting custom storage on FieldStorageConfig

Problem/Motivation

There is a valid use case to support creating configurable fields, whether through the UI or in code, that have custom storage.

With base fields this is simple, we can just call setCustomStorage(TRUE) and we're done. No field tables will get created for that field. However, if you want to implement this using base fields you can, but you would need to take care of your field configuration for instances, and settings etc...

However, this is not possible right now if you are using a configurable field, and the FieldStorageConfig class. hasCustomStorage is hard coded to FALSE. To achieve this currently, you need to alter the field definition in hook_entity_field_storage_info_alter, and wrap the FieldStorageConfig instance with a decorator class to override the custom storage. Like https://github.com/damiankloip/og/blob/audience-field-custom-storage/src...

Proposed resolution

Add $custom_storage as a property and exportable configuration property for FieldStorageConfig objects. We can still default this to FALSE, but allow configurability. In the future we can then apply this to any configurable field easily to suit any use cases.

Remaining tasks

User interface changes

API changes

Data model changes