[contrib.postgres] UUIDField

Added UUIDField. Not particularly complicated this one - no custom lookups or anything. Casts all values to `uuid.UUID` instances in python.

TODO:
- [x] More form field tests
- [x] Documentation

Squashed patch should be marked as fixing #19463.
There is no justification to be this aggressive in the field definition. Making an auto-generating uuid is as simple as `UUIDField(default=uuid.uuid4)`. This should probably be a documented pattern.