Extract scheme-selection policy into a pluggable CostModel #4844

Job	Run time
Rust/C++ FFI tests (asan)	2m 13s
Rust/C++ FFI tests (tsan)	2m 0s
Rust tests (miri)	1m 39s
Rust tests (asan)	6m 47s
Rust tests (tsan)	6m 21s
Mirror DuckDB to R2 / Resolve DuckDB version and check R2	5s
DuckDB libraries available in R2	3s
Mirror DuckDB to R2 / Mirror DuckDB ${{ matrix.archive }} to R2	0s
Rust tests (coverage) (tests)	6m 39s
	25m 47s

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