Request - `count` Restriction to validate the number of items in an array

[joneubank]

Summary of request

The count restriction was proposed in the conditional restrictions spec, but was never implemented. It should be added in to the dictionary spec for fields of all data types. It should also be added into the validation code.

Details

This restriction will validate that an array field has the correct number of entries. The count restriction can be a number, defining the exact number of items required, or a RangeRestriction which defines min/max values.