FEATURE: API to create user's associated account (#15737)

Discourse users and associated accounts are created or updated when a
user logins or connects the account using their account preferences.
This new API can be used to create associated accounts and users too,
if necessary.

spec/requests/api/schemas/json/admin_user_response.json

@@ -486,6 +486,9 @@
           ]
         }
       ]
+    },
+    "external_ids": {
+      "type": "object"
     }
   },
   "required": [
@@ -547,6 +550,7 @@
     "approved_by",
     "suspended_by",
     "silenced_by",
-    "groups"
+    "groups",
+    "external_ids"
   ]
 }

spec/requests/api/schemas/json/user_create_request.json

@@ -21,6 +21,9 @@
     },
     "user_fields[1]": {
       "type": "boolean"
+    },
+    "external_ids": {
+      "type": "object"
     }
   },
   "required": [

spec/requests/api/schemas/json/user_update_request.json

@@ -0,0 +1,17 @@
+{
+  "additionalProperties": false,
+  "properties": {
+    "name": {
+      "type": "string"
+    },
+    "email": {
+      "type": "string"
+    },
+    "password": {
+      "type": "string"
+    },
+    "external_ids": {
+      "type": "object"
+    }
+  }
+}

spec/requests/api/schemas/json/user_update_response.json

@@ -0,0 +1,15 @@
+{
+  "additionalProperties": false,
+  "properties": {
+    "success": {
+      "type": "string"
+    },
+    "user": {
+      "type": "object"
+    }
+  },
+  "required": [
+    "success",
+    "user"
+  ]
+}