Azure.Search: Add strongly typed FacetResult helpers

[tg-msft]

We need to expose richer FacetResult helpers in Track 2. We currently return everything as a dictionary rather than ValueFacetResult and RangeFacetResult. We should decide if we want the As*Facet() approach from Track 1 or if we want to have a class hierarchy here with some best guess duck typed polymorphic deserialization.

[tg-msft]

Need to figure out how to handle the T of As*Facet<T>() if we want to do this automatically.