feat: steppable aggr fn

poc: step aggr query

feat: mvp poc stuff

test: sqlness

chore: import missing

feat: support first/last_value

fix: check also include first/last value

tests/cases/distributed/explain/step_aggr_basic.sql

@@ -0,0 +1,34 @@
+CREATE TABLE integers(
+    host STRING,
+    i BIGINT,
+    ts TIMESTAMP TIME INDEX
+) PARTITION ON COLUMNS (host) (
+    host < '550-A',
+    host >= '550-A'
+    AND host < '550-W',
+    host >= '550-W'
+);
+
+-- count
+EXPLAIN SELECT
+    count(i)
+FROM
+    integers;
+
+EXPLAIN SELECT
+    ts,
+    count(i)
+FROM
+    integers
+GROUP BY
+    ts;
+
+EXPLAIN SELECT
+    date_bin('1 hour'::INTERVAL, ts),
+    count(i)
+FROM
+    integers
+GROUP BY
+    date_bin('1 hour'::INTERVAL, ts);
+
+DROP TABLE integers;