[Provisioning] How to properly handle extensible enums

[ArcturusZhang]

Extracted from code:

azure-sdk-for-net/sdk/provisioning/Azure.Provisioning/src/Expressions/BicepTypeMapping.cs

Line 74 in 349ed73

// TODO: Can we either tag or special case all that we care about because ValueType is too broad

Now we are using ValueType as a final fallback for extensible enums.
Should we do better to distinguish a type is extensible enum?

[ArcturusZhang]

For instance, we could special case all those supported extensible enums from Azure.Core and Azure.ResourceManager
and for those generated by us, we could add a special attribute to it?

if we do not have to be 100% precise, we could add if the type is value type and the type implements the IEquatable<T>