Add support for saml tokens in sp:InitiatorToken

Currently CXF does not support SAML tokens to be used as InitiatorToken in Asymmetric bindings, where as the certificate referred to in the SAML assertion signs the message content (eg SAML Holder of Key scenarios).

chapter 6 Scenario #4 - Holder-of-Key (p28)
http://www.oasis-open.org/committees/download.php/23071/ws-sp-usecases-examples-draft-11-03.doc

chapter 2.3.1.5 (WSS1.0) SAML10 Holder of Key, Sign, Optional Encrypt
http://www.oasis-open.org/committees/download.php/7702/wss-saml-interop1-draft-12.doc

When the <sp:InitiatorToken> contains an <sp:IssuedToken> or a <sp:SamlToken> instead of <sp:WssX509V3Token10>, CXF signs the request and adds a BST by default. CXF does not ask for a SAML token and it is impossible to construct a message signature which SignatureTokenReference contains a reference to the SAML assertion (http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.0#SAMLAssertionID)

<wsse:SecurityTokenReference wsu:id="STR1">
<wsse:KeyIdentifier wsu:id="..."
ValueType="http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.0#SAMLAssertionID">
_a75adf55-01d7-40cc-929f-dbd8372ebdfc
</wsse:KeyIdentifier>
</wsse:SecurityTokenReference>