EntityResource: Provide comprehensive test coverage for FieldStorageConfig entity

Problem/Motivation

Please refer the problem/motivation section of #2737719: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method

Proposed resolution

Write EntityResourceTestBase subclass for the FieldStorageConfig entity.

Unfortunately, FieldStorageConfig does not have an access control handler at all yet, so that needs to be added too. Fortunately, the FieldConfig config entity type has an access control handler that actually looks up the FieldStorageConfig entity that's associated with a FieldConfig entity. So, the logic from that access control handler can be extracted into FieldStorageConfigAccessControlHandler, and then FieldConfigAccessControlHandler can be updated to delegate to FieldStorageConfigAccessControlHandler!

Remaining tasks

1. Create access handler for FieldStorageConfig
2. Write FieldStorageConfigResourceTestBase + subclasses for concrete tests

References

1. Follow-up of #2737719: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method
2. Subtask of #2824572: [PP-14] Write EntityResourceTestBase subclasses for every other entity type.