feat: bind Greptime's own UDF&UDAF into Python Coprocessor Module (#335)

* feat: port own UDF&UDAF into py copr(untest yet)

* refactor: move UDF&UDAF to greptime_builtins

* feat: support List in val2py_obj

* test: some testcases for newly added UDFs

* test: complete test for all added gpdb's own UDF

* refactor: add underscore for long func name

* feat: better error message

* fix: typo

src/common/function/src/scalars/function_registry.rs

@@ -31,6 +31,10 @@ impl FunctionRegistry {
             .insert(func.name(), func);
     }
 
+    pub fn get_aggr_function(&self, name: &str) -> Option<AggregateFunctionMetaRef> {
+        self.aggregate_functions.read().unwrap().get(name).cloned()
+    }
+
     pub fn get_function(&self, name: &str) -> Option<FunctionRef> {
         self.functions.read().unwrap().get(name).cloned()
     }