Represents a base class for SOAP Message Security Policies.
Any WSSPolicy can be epxressed as being composed of one or both of
two SecurityPolicy components called FeatureBinding and KeyBinding.
This generic structure for a WSSPolicy allows for representing complex,
concrete WSS Policy Instances.
For example, A SignaturePolicy can have a SAMLAssertion as its KeyBinding.
The SAMLAssertionBinding can in turn have a KeyBinding which is a PrivateKeyBinding.
The PrivateKeyBinding would contain a PrivateKey corresponding to the PublicKey
contained in the SAML Assertion of the SAMLAssertionBinding. Such a SignaturePolicy
instance can then be used by the XWS-Runtime to sign Message parts of an outgoing
SOAP Message. The MessageParts to be signed are inturn identified by the FeatureBinding
component of the SignaturePolicy.