Edited Issue: Provide api to make Action parameters Nullable [461]

For the following action, I would like to make the parameter 'p3' nullable. Currently, there is no api on the ActionConfiguration which can make it Optional or Nullable.

var actionConfig = vehicle.Action("TestAction1");
actionConfig.Parameter<string>("p1");
actionConfig.Parameter<string>("p2");
actionConfig.Parameter<string>("p3");

Also, from the schema, there seem to be other attributes like "MaxLength", "Precision", "Scale" etc. that can be provided for FunctionImport's parameters:

<xs:attributeGroup name="TFunctionImportParameterAttributes">
<xs:attribute name="Name" type="edm:TSimpleIdentifier" use="required" />
<xs:attribute name="Type" type="edm:TFunctionImportParameterAndReturnType" use="required" />
<xs:attribute name="Mode" type="edm:TParameterMode" use="optional" />
<xs:attribute name="Nullable" type="xs:boolean" use="optional" />
<xs:attribute name="MaxLength" type="edm:TMaxLengthFacet" use="optional" />
<xs:attribute name="Precision" type="edm:TPrecisionFacet" use="optional" />
<xs:attribute name="Scale" type="edm:TScaleFacet" use="optional" />
<xs:attribute name="SRID" type="edm:TSridFacet" use="optional" />
<xs:anyAttribute namespace="##other" processContents="lax" />
</xs:attributeGroup>