Client adapter configuration was updated to support for customization
of HttpClient used for key retrieval similarly to OIDC. Further, it is
now possible to specify several static public keys for signature
verification in saml-client.xml.
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.
