Define IsAlwaysBindable attribute for FunctionImport element

This attribute specifies whether the function can always be bound to entities and consequently need not be advertised in entity representations. Accepted: https://www.oasis-open.org/apps/org/workgroup/odata/download.php/47764/latest/odata-meeting-19_on-20121220-minutes.html

https://tools.oasis-open.org/version-control/browse/wsvn/odata/trunk/spec/schemas/csdl.xsd?rev=129 https://www.oasis-open.org/apps/org/workgroup/odata/download.php/47879/odata-core-v1.0-wd01-part3-csdl-2013-01-11-RH.doc Accepted: https://www.oasis-open.org/committees/download.php/47919/odata-meeting-21_on-20130117-minutes.html#odata-175