Extract scheme-selection policy into a pluggable CostModel #4851

Job	Run time
Rust tests (tsan)	6m 19s
Rust tests (asan)	6m 47s
Rust/C++ FFI tests (asan)	2m 13s
Rust/C++ FFI tests (tsan)	1m 59s
Rust tests (miri)	1m 37s
Mirror DuckDB to R2 / Resolve DuckDB version and check R2	7s
DuckDB libraries available in R2	2s
Mirror DuckDB to R2 / Mirror DuckDB ${{ matrix.archive }} to R2	0s
Rust tests (coverage) (tests)	6m 41s
	25m 45s

Provide feedback