Add SSO `external_ids` to Query User Account admin API (#10261)

Related to #10251

changelog.d/10261.feature

+Add SSO `external_ids` to the Query User Account admin API.

docs/admin_api/user_admin_api.md

@@ -36,7 +36,17 @@ It returns a JSON body like the following:
     "creation_ts": 1560432506,
     "appservice_id": null,
     "consent_server_notice_sent": null,
-    "consent_version": null
+    "consent_version": null,
+    "external_ids": [
+        {
+            "auth_provider": "<provider1>",
+            "external_id": "<user_id_provider_1>"
+        },
+        {
+            "auth_provider": "<provider2>",
+            "external_id": "<user_id_provider_2>"
+        }
+    ]
 }
 ```
 

synapse/handlers/admin.py

@@ -62,9 +62,16 @@ class AdminHandler(BaseHandler):
         if ret:
             profile = await self.store.get_profileinfo(user.localpart)
             threepids = await self.store.user_get_threepids(user.to_string())
+            external_ids = [
+                ({"auth_provider": auth_provider, "external_id": external_id})
+                for auth_provider, external_id in await self.store.get_external_ids_by_user(
+                    user.to_string()
+                )
+            ]
             ret["displayname"] = profile.display_name
             ret["avatar_url"] = profile.avatar_url
             ret["threepids"] = threepids
+            ret["external_ids"] = external_ids
         return ret
 
     async def export_user_data(self, user_id: str, writer: "ExfiltrationWriter") -> Any: