Handle unknown mapping strategies in newer imodels

[RohitPtnkr1996]

WIP

If we have a newer iModel which has a class with a mapping strategy unknown to the current ECDb runtime, the class will be loaded but, however, if we try to query it, we will end up with an "...unsupported map strategy...class wasn't loaded..." error.

Unlike the class modifier or relationship strength fields, a class's mapping strategy does not get checked when the class gets loaded.

This PR adds a new value to the MapStrategy Enum to handle an unknown strategy.
If we try to query this class, we should get a null view even if we are successful in loading the property map (due to the general uncertainty with an unknown mapping strat).

To-Do:

1. Test CTEs to make sure they are working properly.
2. Simulate an unknown mapping strat in an iModel to ensure everything works as expected when loading a property map fails.
3. Increase the testing scope to cover subqueries and joins.

[RohitPtnkr1996]

@khanaffan will you please take a look ?