fix(api): return typed saml form post data in idp intent (#10136)

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# Which Problems Are Solved

The current user V2 API returns a `[]byte` containing a whole HTML
document including the form on `StartIdentifyProviderIntent` for intents
based on form post (e.g. SAML POST bindings). This is not usable for
most clients as they cannot handle that and render a whole page inside
their app.
For redirect based intents, the url to which the client needs to
redirect is returned.

# How the Problems Are Solved

- Changed the returned type to a new `FormData` message containing the
url and a `fields` map.
- internal changes:
- Session.GetAuth now returns an `Auth` interfacce and error instead of
(content string, redirect bool)
- Auth interface has two implementations: `RedirectAuth` and `FormAuth`
- All use of the GetAuth function now type switch on the returned auth
object
- A template has been added to the login UI to execute the form post
automatically (as is).

# Additional Changes

- Some intent integration test did not check the redirect url and were
wrongly configured.

# Additional Context

- relates to zitadel/typescript#410

proto/zitadel/user/v2/idp.proto

@@ -162,3 +162,21 @@ message IDPLink {
     }
   ];
 }
+
+message FormData {
+  // The URL to which the form should be submitted using the POST method.
+  string url = 1 [
+    (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
+      example: "\"https://idp.com/saml/v2/acs\"";
+    }
+  ];
+  // The form fields to be submitted.
+  // Each field is represented as a key-value pair, where the key is the field / input name
+  // and the value is the field / input value.
+  // All fields need to be submitted as is and as input type "text".
+  map<string, string> fields = 2 [
+    (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
+      example: "{\"relayState\":\"state\",\"SAMLRequest\":\"asjfkj3ir2fj248=\"}";
+    }
+ ];
+}

proto/zitadel/user/v2/user_service.proto

@@ -2895,11 +2895,15 @@ message StartIdentityProviderIntentResponse{
         description: "IDP Intent information"
       }
     ];
+    // POST call information
+    // Deprecated: Use form_data instead
     bytes post_form = 4 [
       (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
         description: "POST call information"
       }
     ];
+    // Data for a form POST call
+    FormData form_data = 5;
   }
 }
 

proto/zitadel/user/v2beta/idp.proto

@@ -162,3 +162,21 @@ message IDPLink {
     }
   ];
 }
+
+message FormData {
+  // The URL to which the form should be submitted using the POST method.
+  string url = 1 [
+    (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
+      example: "\"https://idp.com/saml/v2/acs\"";
+    }
+  ];
+  // The form fields to be submitted.
+  // Each field is represented as a key-value pair, where the key is the field / input name
+  // and the value is the field / input value.
+  // All fields need to be submitted as is and as input type "text".
+  map<string, string> fields = 2 [
+    (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
+      example: "{\"relayState\":\"state\",\"SAMLRequest\":\"asjfkj3ir2fj248=\"}";
+    }
+  ];
+}

proto/zitadel/user/v2beta/user_service.proto

@@ -1788,22 +1788,23 @@ message StartIdentityProviderIntentRequest{
 message StartIdentityProviderIntentResponse{
   zitadel.object.v2beta.Details details = 1;
   oneof next_step {
+    // URL to which the client should redirect
     string auth_url = 2 [
       (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
-        description: "URL to which the client should redirect"
         example: "\"https://accounts.google.com/o/oauth2/v2/auth?client_id=clientID&callback=https%3A%2F%2Fzitadel.cloud%2Fidps%2Fcallback\"";
       }
     ];
-    IDPIntent idp_intent = 3 [
-      (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
-        description: "IDP Intent information"
-      }
-    ];
+    // IDP Intent information
+    IDPIntent idp_intent = 3;
+    // POST call information
+    // Deprecated: Use form_data instead
     bytes post_form = 4 [
       (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
         description: "POST call information"
       }
     ];
+    // Data for a form POST call
+    FormData form_data = 5;
   }
 }