Test SAML integration

[christophe-f]

What needs to be done?

Test that Showcase is working with SAML

https://github.com/backstage/backstage/blob/b1f4c99f79f2640ae3f12139923850c47f61aa4c/plugins/auth-backend/scripts/start-saml-idp.sh

[Zaperex]

Currently SAML is not supported in the showcase because the showcase does not have the SAML auth provider setup.
https://backstage.io/docs/reference/plugin-auth-backend.providers/

We also currently don't have a specific sign-in page for SAML yet, which would require a custom apiRef and sign-in configuration like the following:

{
  id: 'saml-auth-provider',
  title: 'SAML',
  message: 'Sign in using SAML',
  apiRef: samlAuthApiRef,
},

export const samlAuthApiRef: ApiRef<
  OAuthApi &
    OpenIdConnectApi &
    ProfileInfoApi &
    BackstageIdentityApi &
    SessionApi
> = createApiRef({
  id: 'internal.auth.saml',
});

Then we would register the API similar to how @schultzp2020 registered the OIDC apiRef in #602 in packages/app/src/api/AuthApiRefs.ts

// SAML
export const apis: AnyApiFactory[] = [
  ...
  createApiFactory({
    api: samlAuthApiRef,
    deps: {
      discoveryApi: discoveryApiRef,
      oauthRequestApi: oauthRequestApiRef,
      configApi: configApiRef,
    },
    factory: ({ discoveryApi, oauthRequestApi, configApi }) =>
      OAuth2.create({
	discoveryApi,
	oauthRequestApi,
	provider: {
	  id: 'saml',
	  title: 'SAML',
	  icon: () => null,
	},
	environment: configApi.getOptionalString('auth.environment'),
      }),
  }),
];

The users would then need to also configure the saml provider in the auth.providers section of the app-config.yaml according to the schema specified by the config.d.ts which currently doesn't have much documentation on what's expected as inputs.

[schultzp2020]

@christophe-f @Zaperex I will add saml support in #602.

[Zaperex]

Currently blocked by #602 and #588

[Zaperex]

Verified and the SAML sign-in works as expected.