Add user_defined_sql_planners(..) to FunctionRegistry #28917

Job	Run time
build with wasm-pack	2m 18s
cargo test (macos)	10m 49s
cargo check	8m 9s
cargo test (win64)	33m 28s
Check cargo fmt	40s
cargo test (macos-aarch64)	8m 43s
Verify MSRV (Min Supported Rust Version)	6m 26s
Verify Vendored Code	57s
cargo test doc (amd64)	12m 57s
cargo test datafusion-cli (amd64)	3m 44s
cargo test (amd64)	9m 14s
cargo examples (amd64)	12m 40s
clippy	5m 5s
check Cargo.toml formatting	2m 52s
check configs.md is up-to-date	2m 29s
cargo test hash collisions (amd64)	8m 57s
cargo doc	3m 33s
Run sqllogictest with Postgres runner	3m 3s
cargo test pyarrow (amd64)	2m 40s
verify benchmark results (amd64)	10m 32s
	2h 29m 16s

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