RFC: Make fields of ScalarUDF non pub #19816

Job	Run time
Check cargo fmt	55s
cargo check	2m 33s
build with wasm-pack	2m 40s
Verify MSRV	14m 35s
cargo test (mac)	19m 1s
Verify Vendored Code	45s
cargo test (win64)	36m 11s
cargo test (amd64)	8m 35s
cargo examples (amd64)	4m 35s
check Cargo.toml formatting	1m 58s
cargo test hash collisions (amd64)	8m 32s
cargo doc	6m 2s
cargo test datafusion-cli (amd64)	5m 28s
cargo test doc (amd64)	15m 26s
Run sqllogictest with Postgres runner	3m 1s
clippy	4m 21s
cargo test pyarrow (amd64)	3m 21s
verify benchmark results (amd64)	17m 0s
check configs.md is up-to-date	4m 14s
	2h 39m 13s

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