Cosmos: Evaluate the data types in server side to see need of convert node

[smitpatel]

CosmosDb doesn't have a function to convert/cast to other data type like relational providers so current Convert nodes in LINQ cannot be translated. Though certain nodes may not require conversion due to implicit conversion/storage mechanism in server side (e.g. float to double conversion). We need to evaluate such implicit conversions in CosmosDb and pass through such conversions in VisitUnary as no-op.