streams: Add endpoint "GET /streams/{stream_id}" to get stream by id.

Fixes #22082.

zerver/views/streams.py

@@ -754,6 +754,16 @@ def get_streams_backend(
     return json_success(request, data={"streams": streams})
 
 
+@has_request_variables
+def get_stream_backend(
+    request: HttpRequest,
+    user_profile: UserProfile,
+    stream_id: int,
+) -> HttpResponse:
+    (stream, sub) = access_stream_by_id(user_profile, stream_id, allow_realm_admin=True)
+    return json_success(request, data={"stream": stream.to_dict()})
+
+
 @has_request_variables
 def get_topics_backend(
     request: HttpRequest,