Add `arrow_metadata(<expr>)` UDF

[adriangb]

As discussed in #19335 it would be nice to have a function arrow_metadata similar to arrow_typeof that returns the metadata from it's input field as a Map<String, String> column. Ideally it could support two signatures arrow_metadata(column_or_expr) -> Map<String, String> and arrow_metadata(column_or_expr, 'key') -> String.

[Jefffrey]

Existing reference:

datafusion/datafusion/sqllogictest/src/test_context.rs

Lines 406 to 453 in 2b67ba8

	/// UDF to extract metadata from a field for testing purposes
	/// Usage: get_metadata(expr, 'key') -> returns the metadata value or NULL
	#[derive(Debug, PartialEq, Eq, Hash)]
	struct GetMetadataUdf {
	signature: Signature,
	}
	impl GetMetadataUdf {
	fn new() -> Self {
	Self {
	signature: Signature::any(2, Volatility::Immutable),
	}
	}
	}
	impl ScalarUDFImpl for GetMetadataUdf {
	fn as_any(&self) -> &dyn Any {
	self
	}
	fn name(&self) -> &str {
	"get_metadata"
	}
	fn signature(&self) -> &Signature {
	&self.signature
	}
	fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
	Ok(DataType::Utf8)
	}
	fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
	// Get the metadata key from the second argument (must be a string literal)
	let key = match &args.args[1] {
	ColumnarValue::Scalar(ScalarValue::Utf8(Some(k))) => k.clone(),
	_ => {
	return exec_err!("get_metadata second argument must be a string literal")
	}
	};
	// Get metadata from the first argument's field
	let metadata_value = args.arg_fields[0].metadata().get(&key).cloned();
	// Return as a scalar (same value for all rows)
	Ok(ColumnarValue::Scalar(ScalarValue::Utf8(metadata_value)))
	}
	}

As part of this issue, we should remove the above UDF and use the new arrow_metadata() UDF directly in the tests