Add the vot to the SAML response when it is used as an AuthnContext for SAML

[jmhooper]

In #10178 we added the ability for SAML service providers to make a request with a vector of trust in the AuthnContext. When a SAML SP does this the vector of trust that is used for the authentication transaction should be reflected to the SP in the SAML response.

The authentication context appears in 2 places in the SAML request:

1. In the authn context for the entire transaction:

   <AuthnStatement AuthnInstant="2024-01-01T00:00:00" SessionIndex="_abc-123-def-456">
     <AuthnContext>
       <AuthnContextClassRef>A1.B2.C3</AuthnContextClassRef>
     </AuthnContext>
   </AuthnStatement>

2. In the attribute statement. With ACR values these appeared as seperated AAL and IAL nodes. For VTRs they appear in a single VTR node:

   <AttributeStatement>
     <!-- ... -->
     <Attribute Name="vot" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri" FriendlyName="vot">
       <AttributeValue>A1.B2.C3</AttributeValue>
     </Attribute>
   </AttributeStatement>

Making the VoT appear like this required changes in 2 places:

1. In the SAML controller the correct authn_context value was passed to the #encode_response method. This is a method from the 18f/saml_idp gem which is overriden in SamlIdpController.
2. The AttributeAsserter was modified to recognized a VTR request and add the correct values to the AttributeStatement node in the SAML response.