Add display name to the Extenstions of SAML request

Add DisplayName to the <samlp:Extensions> of the Stepup callout AuthnRequest

User story
As a user, I want to see the name of the service I am logging into during second-factor authentication, so that I can verify I am authenticating for the right application and detect unexpected or malicious authentication attempts.

Background
This is the EB side of a broader initiative to show the service name during Stepup authentication. See the RFC for the full design: OpenConext/Stepup-Gateway#587.

EB acts as an SFO application connecting to Stepup-Gateway on behalf of thousands of services. Rather than showing a generic proxy name (e.g. "OpenConext"), we want to pass the actual service name to Stepup so it can be shown in the 2FA UI, push notifications, and SMS messages.

EB already knows the service name via the name:en and name:nl fields of the SP entity (saml20_sp) or RP entity (oidc10_rp) from OpenConext-manage, pushed to EB via the metadata push API.

What needs to be done

Introduce a feature flag feature_stepup_send_service_name in EB (following the existing feature flag pattern in config/packages/parameters.yml.dist).

When enabled, for every Stepup callout AuthnRequest, EB MUST add an mdui:UIInfo element to the <samlp:Extensions> containing a mdui:DisplayName for each available language. Example:

<samlp:Extensions>
  <mdui:UIInfo>
    <mdui:DisplayName xml:lang="nl">Electronische leeromgeving</mdui:DisplayName>
  </mdui:UIInfo>
</samlp:Extensions>

Acceptance criteria

• A feature flag feature_stepup_send_service_name is introduced, disabled by default
• When enabled, the mdui:UIInfo element with mdui:DisplayName entries for en and nl is added to the <samlp:Extensions> of the Stepup callout AuthnRequest
• The display names are sourced from name:en and name:nl or any other locale of the SP/RP entity from OpenConext-manage
• If a language's display name is not available, that mdui:DisplayName element is omitted (no empty values)
• When the feature flag is disabled, the AuthnRequest is unchanged from current behavior

Related

• RFC: OpenConext/Stepup-Gateway#587