Provide config schema to field types and storage in datetime module

Follow up for #1953404: Add config schema to field and instance config entities

Problem/motivation

#1866610: Introduce Kwalify-inspired schema format for configuration introduced the idea of config schema. The changelog leads to (hopefully extensive) documentation on the format at http://drupal.org/node/1905070. As a part of schema addition to field module(#1953404: Add config schema to field and instance config entities), found that we have to provide schema for field types, field widgets and field storage.

Proposed solution

Create a configuration schema for field types, field widget and field storage (if applicable) defined in datetime module.
Schema in place

Schema not yet in place
field.datetime.settings
field.datetime.instance_settings
field.datetime.value

field_widget.datetime_default.value
field_widget.datetime_datelist.value