Convert `VariancePopulation` to UDAF #27704

Job	Run time
cargo check	8m 6s
Check cargo fmt	36s
build with wasm-pack	2m 13s
cargo test (macos)	10m 31s
cargo test (macos-aarch64)	9m 16s
Verify MSRV (Min Supported Rust Version)	6m 58s
Verify Vendored Code	44s
cargo test (win64)	33m 14s
cargo examples (amd64)	12m 40s
cargo doc	3m 39s
cargo test (amd64)	9m 26s
cargo test datafusion-cli (amd64)	3m 39s
cargo test doc (amd64)	11m 55s
Run sqllogictest with Postgres runner	3m 14s
cargo test pyarrow (amd64)	2m 32s
verify benchmark results (amd64)	10m 33s
check Cargo.toml formatting	2m 38s
clippy	5m 40s
check configs.md is up-to-date	2m 29s
cargo test hash collisions (amd64)	8m 40s
	2h 28m 43s

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