Returns the set of Supported-Features AVPs. The returned array contains
the AVPs in the order they appear in the message.
A return value of null implies that no Supported-Features AVPs have been set.
The elements in the given array are SupportedFeatures objects.
Sets the set of Supported-Features AVPs, with all the values in the given array.
The AVPs will be added to message in the order in which they appear in the array.
Note: the array must not be altered by the caller following this call, and
getSupportedFeatureses() is not guaranteed to return the same array instance,
e.g. an "==" check would fail.
java.lang.IllegalStateException - if setSupportedFeatures or setSupportedFeatureses
has already been called