Spring Security Add support for AuthnRequestsSigned setting

Expected Behavior

https://docs.oasis-open.org/security/saml/v2.0/saml-metadata-2.0-os.pdf#740 defines it. It is useful when a service provider wants to sign the request no matter what.

Current Behavior Right now the SAML request is signed only if the asserting party has set WantAuthnRequestsSigned.

Context Trying to migrate from the old SAML library and we use AuthnRequestsSigned setting to always sign the request.