Implement SHOW FUNCTIONS #31607

Job	Run time
Check cargo fmt	53s
cargo check	7m 17s
build with wasm-pack	2m 23s
cargo test (macos)	10m 31s
cargo test (macos-aarch64)	9m 41s
Verify MSRV (Min Supported Rust Version)	6m 41s
Verify Vendored Code	49s
cargo test (win64)	43m 53s
cargo examples (amd64)	13m 9s
cargo test (amd64)	9m 37s
cargo test datafusion-cli (amd64)	3m 57s
check Cargo.toml formatting	2m 57s
clippy	5m 51s
check configs.md is up-to-date	2m 49s
cargo test hash collisions (amd64)	13m 57s
cargo doc	3m 51s
cargo test doc (amd64)	14m 21s
Run sqllogictest with Postgres runner	3m 24s
cargo test pyarrow (amd64)	2m 47s
verify benchmark results (amd64)	11m 54s
	2h 50m 42s

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