chore: add dbbenchmark grouby benchmark q8 (#796)

Author: Genesis929

scripts/benchmark/db-benchmark/groupby/G1_1e9_1e2_5_0/q8.py

@@ -0,0 +1,19 @@
+# Contains code from https://github.com/duckdblabs/db-benchmark/blob/master/pandas/groupby-pandas.py
+
+import bigframes.pandas as bpd
+
+print("Groupby benchmark 8: largest two v3 by id6")
+
+x = bpd.read_gbq("bigframes-dev-perf.dbbenchmark.G1_1e9_1e2_5_0")
+
+ans = (
+    x[~x["v3"].isna()][["id6", "v3"]]
+    .sort_values("v3", ascending=False)
+    .groupby("id6", as_index=False, dropna=False)
+    .head(2)
+)
+print(ans.shape)
+chk = [ans["v3"].sum()]
+print(chk)
+
+bpd.reset_session()