Issue duckdb#12600: Streaming Positive LAG

Use buffering to support streaming computation of constant positive LAGs
and negative LEADs that are at most one vector away.
This doesn't fix the "look ahead" problem, but the benchmark shows
it is about 5x faster than the non-streaming version.

benchmark/micro/window/streaming_lag.benchmark

@@ -0,0 +1,27 @@
+# name: benchmark/micro/window/streaming_lag.benchmark
+# description: Verify performance of streaming LAG
+# group: [window]
+
+load
+SELECT SETSEED(0.8675309);
+CREATE OR REPLACE TABLE df AS
+	SELECT 
+		RANDOM() AS a,
+		RANDOM() AS b,
+		RANDOM() AS c,
+	FROM range(10_000_000);
+
+run
+SELECT sum(a_1 + a_2 + b_1 + b_2)
+FROM (
+	SELECT
+	  LAG(a, 1) OVER () AS a_1,
+	  LAG(a, 2) OVER () AS a_2,
+	  LAG(b, 1) OVER () AS b_1,
+	  LAG(b, 2) OVER () AS b_2
+	FROM df
+) t
+;
+
+result I
+20000902.549240764