Open up traits for consumption by extensions

[Evangelink]

As of now, when using traits with MTP you have 2 options:

• MetadataProperty - used by TUnit, xUnit v3, TATF
• the internal serializable kvp property - used by the bridge

When an extension wants to act on this property it currently can do it only on MetadataProperty which won't work for VSTest bridge extensions (MSTest, NUnit...). The bridge should be changed to no longer use the serializable property but instead rely on the MetadataProperty.

Ideally, in order to reduce memory consumption, these properties should not always be emitted by the test framework and we should have a capability like we have for TRX that would allow clients or extensions to declare they are interested about such properties so that test framework can conditionally emit them.