Allow cast of dynamic operand in queries

Standard JCR-SQL2 only allows `CAST( ``value`` AS ``type`` )` on the right-hand side of a constraint. This is limiting, especially when the property value may contain a value that is convertible to another type and the constraint should be applied using that other type.

For example, consider a node type with a property definition for `propertyY` that defines the type as a STRING. Even if some of those values contained string-representations of DOUBLE values, the following query could not be performed:

SELECT * FROM [nodex] as x  where x.propertyY <= 20.50

It would be nice to be able to do this:

SELECT * FROM [nodex] as x  where CAST(x.propertyY AS DOUBLE) <= 20.50

Doing this will require having a `DynamicOperand` subtype for the `CAST` operation, though the value in the cast probably only needs to be a `PropertyValue`. Specifically, something like this:

DynamicOperand ::= PropertyValue | ReferenceValue |
                   Length | NodeName | NodeLocalName |
                   NodePath | NodeDepth |
                   CastPropertyValue |
                   FullTextSearchScore |
                   LowerCase | UpperCase |
                   Arithmetic |
                   '(' DynamicOperand ')'

CastPropertyValue ::= 'CAST(' PropertyValue ' AS ' PropertyType ')'

where `PropertyValue` and `PropertyType` are already defined in the grammar.