Add LAG window function

[Ulimo]

This fixes #769

[Copilot]

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/FlowtideDotNet.Substrait/Sql/Internal/BuiltInSqlFunctions.cs:970

• [nitpick] Consider rewording the error message thrown here to 'lag requires at least one argument and cannot be a wildcard' for improved clarity.

if (argList.Args == null || argList.Args.Count < 1)

src/FlowtideDotNet.Substrait/Sql/Internal/BuiltInSqlFunctions.cs:974

• [nitpick] Consider updating the error message to clearly state that a wildcard argument is not permitted for lag; this will aid in debugging.

if ((argList.Args[0] is FunctionArg.Unnamed unnamed0 && unnamed0.FunctionArgExpression is FunctionArgExpression.Wildcard))

[github-actions]

Benchmark

Benchmark suite	Current: 50102f3	Previous: 22d4209	Ratio
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.InnerJoin	409783200 ns (± 6364744.931260011)	458826533.3333333 ns (± 10640413.19005047)	0.89
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.LeftJoin	514421750 ns (± 26031329.902905248)	564994640 ns (± 28550338.329818636)	0.91
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.ProjectionAndNormalization	148660140 ns (± 5302674.420296486)	169842820 ns (± 7144067.768715523)	0.88
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.SumAggregation	153088630 ns (± 17961706.650464296)	171581250 ns (± 12415362.812660772)	0.89
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.ListAggWithMapAggregation	1838919370 ns (± 95276724.66238132)	1982296640 ns (± 123349811.75190067)	0.93
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.WindowSum	360495483.3333333 ns (± 17292831.922071062)
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.ListAggWithStructAggregation	1458525580 ns (± 95911366.25078848)

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