Add support for AuthnRequestsSigned setting

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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.