chore: add function to set prefer_existing_union (#10322) #108

Job	Run time
cargo test (macos)	11m 1s
Check cargo fmt	43s
build with wasm-pack	2m 10s
cargo check	4m 22s
cargo test (macos-aarch64)	9m 16s
Verify MSRV (Min Supported Rust Version)	6m 24s
Verify Vendored Code	56s
cargo test (win64)	35m 22s
cargo examples (amd64)	10m 55s
cargo doc	3m 31s
cargo test datafusion-cli (amd64)	3m 38s
cargo test (amd64)	10m 27s
Run sqllogictest with Postgres runner	3m 12s
cargo test doc (amd64)	11m 37s
verify benchmark results (amd64)	10m 32s
clippy	4m 48s
cargo test pyarrow (amd64)	2m 43s
check Cargo.toml formatting	2m 54s
check configs.md is up-to-date	2m 42s
cargo test hash collisions (amd64)	9m 47s
	2h 27m 0s

Provide feedback