public BaseFieldDefinition::getPropertyNames()
Returns the names of the field's subproperties.
A field is a list of items, and each item can contain one or more properties. All items for a given field contain the same property names, but the values can be different for each item.
For example, an email field might just contain a single 'value' property, while a link field might contain 'title' and 'url' properties, and a text field might contain 'value', 'summary', and 'format' properties.
Retur