Fix get type for nullable enums and fix WUX projections

[manodasanW]

• In the previous PR for nullable, we fixed the scenario where WinUI asks for the type of the nullable version of built-in types. But it looks common for WinUI to do the same for nullable enums too. This expands the approach we take with WinRTExposedType to enums and addresses this issue via an interface that it internally implements to get the nullable version of the enum.
• Also removing the throw we had in ResolveGenericType as we always catch that exception later and return null. It was there primarily as a hint, but given we now properly support nullable in this scenario and we have fallbacks for the other scenarios such as during RCW creation, removing it.
• Noticed that the custom mapped WUX projections were missing the AOT attributes for WinRTExposedType, so adding them there.

Fixes #1697
Fixes #1702
Fixes #1623

[manodasanW]

Working on doing a test to see if there is much impact to the size.