Added options to switch IdP configurations such as x509cert, secret key and password on the fly

[hamaron]

I've had this feature in my fork and used it in Production for years, so I wanted to give it back to the community.

Just as #88, we want to change the IdP configuration such as x509cert, secret key, and password on the fly if the IDP is multitenancy, depending on who the customer is.

You can change the configuration when returning a SAML response like so:

def create
        @saml_response = encode_response(current_user, {
          x509_certificate: "=== Valid Certificate ===",
          secret_key: "Valid Secret Key"
        })
        render template: "saml_idp/idp/saml_post", layout: false
end

[Zogoo]

@hamaron thanks for your contribution, The code looks good to me.
can you resolve the conflict in your branch?

[Zogoo]

This pull request has been recreated as #209. We will consider that pull request as the latest version since this one is outdated.