frontend: udf looks bad in EXPLAIN

[BugenZhao]

We directly use the Debug impl of UserDefinedFunction when explaining:

risingwave/src/frontend/src/expr/mod.rs

Line 1144 in ac966f1

ExprImpl::UserDefinedFunction(x) => write!(f, "{:?}", x),

...which includes the FunctionCatalog, and even the WASM binary as a bytes array!

risingwave/src/frontend/src/catalog/function_catalog.rs

Lines 25 to 42 in ac966f1

	#[derive(Clone, PartialEq, Eq, Hash, Debug)]
	pub struct FunctionCatalog {
	pub id: FunctionId,
	pub name: String,
	pub owner: u32,
	pub kind: FunctionKind,
	pub arg_names: Vec<String>,
	pub arg_types: Vec<DataType>,
	pub return_type: DataType,
	pub language: String,
	pub identifier: Option<String>,
	pub body: Option<String>,
	pub link: Option<String>,
	pub compressed_binary: Option<Vec<u8>>,
	pub always_retry_on_network_error: bool,
	pub function_type: Option<String>,
	pub runtime: Option<String>,
	}

For example,

create function gcd(int, int) returns int language rust as $$
    fn gcd(mut a: i32, mut b: i32) -> i32 {
        while b != 0 {
            (a, b) = (b, a % b);
        }
        a
    }
$$;

explain select gcd(6, 9);

will leads to an output of 3.6 MB.