Allow less strict type casting when a type doesn't exist

[Archomeda]

Currently when a CastableType doesn't exist, Gw2Sharp throws an exception like System.Text.Json.JsonException: Unsupported type Quux. This is unhelpful when new content gets added, because this requires an update of Gw2Sharp before stuff works again.

This checking should be less strict. If a type doesn't exist, the base class should be used instead. There should be an option to enable strict type casting (useful for testing), but the default should be set to false.

Ref #117