Contrary to POST binding, signature of SAML protocol message sent using
REDIRECT binding is contained in query parameters and not in the
message. This renders <dsig:KeyName> key ID hint unusable. This commit
adds <Extensions> element in SAML protocol message containing key ID so
that key ID is present in the SAML protocol message.
Changes of SAML assertion creation/parsing that are required to allow
for validation of rotating realm key: signed SAML assertions and signed
SAML protocol message now contain signing key ID in XML <dsig:KeyName>
element.
