Change elixir to use auth code flow
Change elixir to use auth code flow

Also move non-OIDC flow behind Experimental flags

PiperOrigin-RevId: 295182544
Change-Id: I018674c29c28773f4184deeb1f4e71b806cd9045

GoogleCloudPlatform/healthcare-federated-access-services,GoogleCloudPlatform/healthcare-federated-access-services,GoogleCloudPlatform/healthcare-federated-access-services,GoogleCloudPlatform/healthcare-federated-access-services