Expose field storage in API

I think it would be helpful to expose the field storage configuration also as part of API, so that API consumer can be aware of storage specific configs like cardinality, options for select list.

Proposed Solution:
in src/Plugin/rest/resource/EntityBundleFieldsResource.php
- $fields[$field_instance->getName()] = $field_instance;
+ $field_storage = $field_instance->getFieldStorageDefinition();
+ $fields[$field_instance->getName()]['field_config'] = $field_instance;
+ $fields[$field_instance->getName()]['field_storage'] = $field_storage;