Support generators for PathPropertiesConstraint and PropertyValueConstraint

[robinmackaij]

Current implementation accepts lists and randomly picks an item from the list to use in the request. This means that the same value can be used multiple times during a test cycle. In some cases, this may cause issues. Supporting generators in addition to lists would allow library users to prevent such duplications.

Note: the library user will be responsible for ensuring the provided generator is not exhausted before the test suite completes.
In the advanced_use documentation, the use of itertools.cycle() should be suggested as a way to ensure all provided values are used while preventing StopIteration exceptions during test runs.