SqlContentEntityStorage::getFieldStorageDefinitions() is poorly named and should not be public

What are the steps required to reproduce the bug?

1. Create a custom ContentEntityType.
2. Attach at least one storage definition to this ContentEntityType using the hook 'entity_field_storage_info'.
3. Create required database fields accordingly to the attached storage definition.
4. Create an instance of that entity and try to save it.

What behavior were you expecting?

What happened instead?

Cause

Workaround

This is expected behavior. Storage Definitions in hook_field_storage_info do not go in the base tables and should throw the exception. Rename the confusing function and make it protected with appropriate deprecations and tests.