refactor: Consolidate single group by column code into sub modules (#… #103

Job	Run time
cargo check	2m 7s
build with wasm-pack	2m 10s
Check License Header	2m 12s
cargo test (macos)	2m 22s
cargo test (macos-aarch64)	2m 21s
Verify Vendored Code	52s
Check cargo fmt	43s
cargo test (win64)	2m 11s
Verify MSRV (Min Supported Rust Version)	2m 9s
clippy	1s
check Cargo.toml formatting	1s
cargo test hash collisions (amd64)	1s
check configs.md and ***_functions.md is up-to-date	1s
cargo doc	1s
cargo test datafusion-cli (amd64)	1s
cargo test doc (amd64)	1s
cargo test (amd64)	1s
cargo examples (amd64)	1s
Run sqllogictest with Postgres runner	1s
cargo test pyarrow (amd64)	1s
verify benchmark results (amd64)	1s
	17m 19s

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