This commit enables SAML service providers to make requests using vectors of trust. Vectors of trust support has been built into OIDC in previous commits and much of the work to support this feature was done there.

This commit usee the changes in 18F/saml_idp#91 to capture the VTR and store it in the service provider request (and eventually `sp_session`). This is done in the `FederateProtocols::Saml` model similar to the approach for OIDC.

This commit modifies `SamlRequestValidator` to validate VTRs alongside validations that exist today for `AuthnContextClassref`s. The `Vot::Parser` is used here to validate the format of VoTs. The `SamlRequestValidator` validates that an SP is allowed to make requests with identity proofing if it is requested. Finally the `SamlRequestValidator` will also fail if the vector or trust feature is not available in the environment

The logic for biometric comparison was added to `SamlIdpController` to support the `Pb` component-value that can be used there now. Users are prompted to go through proofing with a selfie if it is required. This includes users who have proofed without a selfie.. If the biometric comparison feature is not enabled for the environment and it is requested with the `Pb` component value `SamlIdpController` will render a 406 similar to the `OpenidConnect::AuthorizationController`.

changelog: Internal, SAML, VTR support was added to SAML