Split SplitBy::Layout into Layout and LayoutSubSplitting variants #6588

Job	Run time
Rust tests (miri)	1m 34s
C++ API (coverage)	1m 7s
Rust tests (asan)	6m 58s
Rust/C++ FFI tests (asan)	2m 10s
Rust tests (tsan)	6m 37s
Rust/C++ FFI tests (tsan)	1m 54s
Mirror DuckDB to R2 / Resolve DuckDB version and check R2	8s
DuckDB libraries available in R2	4s
Mirror DuckDB to R2 / Mirror DuckDB ${{ matrix.archive }} to R2	0s
Rust tests (coverage) (tests)	6m 33s
	27m 5s

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