sax: FastAttributeList - add attribute in a specific namespace

This change is inspired by startElementNS & singleElementNS which
have namespace as first parameter to define the namespace of an
element. Some attributes of a element can be in different namespace
but until now FastAttributeList "add" method did not have a namespace
parameter. This commit adds "addNS" which accepts namespace as a
first parameter.

Change-Id: Iebf8b5e890c17f00a56923efc3506580eed070a9