Cosmos: Support aggregate operators

[smitpatel]

Average/Sum/Min/Max/Count

 typeof(Microsoft.Azure.Cosmos.QueryPlanHandler+QueryPlanHandlerException): Query contains 1 or more unsupported features. Upgrade your SDK to a version that does support the requested features:
    Query contained CompositeAggregate, which the calling client does not support. (Query contained CompositeAggregate, which the calling client does not support.)
    ---- Microsoft.Azure.Cosmos.QueryPlanHandler+QueryPlanHandlerException : Query contains 1 or more unsupported features. Upgrade your SDK to a version that does support the requested features:
    Query contained CompositeAggregate, which the calling client does not support. (Query contained CompositeAggregate, which the calling client does not support.)
    -------- System.ArgumentException : Query contained CompositeAggregate, which the calling client does not support.

[smitpatel]

https://docs.microsoft.com/en-us/azure/cosmos-db/sql-query-linq-to-sql