Support for _drop_dynamic_properties

[gastlich]

Hi there,
it would be nice, if there was a way of dropping dynamic properties instead of allowing them or not.

Currently the default behaviour is accepting dynamic properties (_allow_dynamic_properties = True).
By changing this attribute, the library raises an error, and this is a valid approach in a case when you would like to be very strict about your schema. In our case we would like to be very strict on things we defined in the schema manually, but drop all dynamic fields additionally.