refactor(webapi,types)!: make challenge type extensive

We may allow more challenge types other than PoW in the future, eg.
captcha. So make the relevent types more generic.

Now the challenge is returned in JSON response as a individual top-level
field `register_challenge` instead of in HTTP headers.

docs/webapi.yaml

@@ -85,17 +85,10 @@ paths:
         404:
           description: |
             The user is not registered, or no token is not provided.
-          headers:
-            x-blah-nonce:
-              description: The challenge nonce for registration.
+          content:
+            application/json:
               schema:
-                type: integer
-                format: uint32
-            x-blah-difficulty:
-              description: The challenge difficulty for registration.
-              schema:
-                type: integer
-                format: uint32
+                $ref: '#/components/schemas/ApiErrorWithRegisterChallenge'
 
     post:
       summary: Register or update user identity
@@ -554,6 +547,24 @@ components:
               description: A human-readable error message.
               example: signature verification failed
 
+    ApiErrorWithRegisterChallenge:
+      allOf:
+        - $ref: '#/components/schemas/ApiError'
+        - type: object
+          properties:
+            register_challenge:
+              type: object
+              properties:
+                pow:
+                  type: object
+                  properties:
+                    nonce:
+                      type: integer
+                      format: uint32
+                    difficulty:
+                      type: integer
+                      format: uint32
+
     RoomList:
       type: object
       required:
@@ -869,10 +880,16 @@ components:
                 id_key:
                   type: string
                   description: Hex encoded user primary key `id_key`.
-                challenge_nonce:
-                  type: integer
-                  format: uint32
-                  description: The challenge nonce retrieved from a recent GET response of `/user/me`.
+                challenge:
+                  type: object
+                  properties:
+                    pow:
+                      type: object
+                      properties:
+                        nonce:
+                          type: integer
+                          format: uint32
+                          description: The challenge nonce retrieved from a recent GET response of `/user/me`.
 
     UserIdentityDescription:
       type: object