OrderBy doesn't support all expression defined in OData-ABNF

Motivation:

I'm using OData library to get access to dynamic properties of my entity. Look #304 issue for details. I would like to execute queries like https://endpoint.ua/odata/entity?$orderby=cast(metadata/myValue, Edm.Int32). Currently OrderBy nodes can process only SingleValueOpenPropertyAccessNode or SingleValuePropertyAccessNode. It means that query in my example is considered invalid. However OData construction rules defines more valid expressions for orderby operation. Here is a part of it

orderby = ( "$orderby" / "orderby" ) EQ orderbyItem *( COMMA orderbyItem )
orderbyItem = commonExpr [ RWS ( "asc" / "desc" ) ]

And commonExpr might be castExpr which is cast function call. Some providers might not be able to translate it, but databases supports queries like this (postgres has CAST (field AS type) expression and MsSQL has similar option). But there should be ability to properly process query in case it might be executed.

Change proposal

Allow to pass commonExpr in orderby query. Which requires modification of OrderByNode class (to support more child nodes in this query).

Start by inspecting the OrderByNode class and the orderby parsing and validation path. Compare the currently accepted SingleValueOpenPropertyAccessNode and SingleValuePropertyAccessNode cases with the cast expression described in the issue and review existing tests for orderby expressions. Done means valid commonExpr expressions such as cast(metadata/myValue, Edm.Int32) are accepted without breaking existing orderby behavior.