Change field types to define their property definitions statically

As discussed with @fago in Vienna:

ComplexDataInterface::getPropertyDefinitions() is currently a non-static method on Data objects, meaning it can potentially vary object by object and depend on the actual values being held. For field types, that would be very weird.
We discussed that getPropertyDefinitions() should only depend on the "item definition", and thus be a static method receiving a FieldDefinition object.

Then, there's the question of the static::$propertyDefinitions pattern currently used in there.
This is done to avoid recreating the property definitions on each call, and each field type structures its static::$properties according to the granularity by which the definitions may vary. This is slightly obscure and can get a bit complex to follow - see ConfigEntityReferenceItemBase::getPropertyDefinitions().

The way this is currently done with getSchema() is that FieldItem::getSchema() just determines the schema array and returns it; The "optimization/caching" part is not the responsibility of the FieldItem class, but is done inside the FieldDefinition object (Field::getSchema()). Meaning, the schema is "statically cached" by each FieldDefinition, and code that needs the schema for a given field asks it to the FieldDefinition rather than to a FieldItem object.
Could a similar pattern be used here ?