Add configuration schema for Single Date Time Picker field widgets

This is still relevant on the 2.x branch.

Patch still needs work:

1. It is missing a schema definition for single_date_time_timestamp_widget
2. Is it missing a schema definition for the following properties: allow_seconds, start_date, allow_blank

1. The single_date_time_timestamp_widget has no configuration, so no config schema is required
2. Added a config schema for the properties: allow_seconds, start_date, allow_blank

I have tested it, it works as expected.

Automatically closed - issue fixed for 2 weeks with no activity.