Add object type (#1948)

Add a new type: Object. Currently, it's mainly for complex aggregate metrics(HLL , Bitmap).

The Object type has the following constraints：
1 Object type could not as key column type
2 Object type doesn't support all indices (BloomFilter, short key, zone map, invert index)
3 Object type doesn't support filter and group by

In the implementation：

The Object type reuse the StringValue and StringVal, because in storage engine, the Object type is binary, it has a pointer and length.

be/src/exprs/case_expr.cpp

@@ -327,6 +327,7 @@ void CaseExpr::get_child_val(int child_idx, ExprContext* ctx, TupleRow* row, Any
     case TYPE_CHAR:
     case TYPE_VARCHAR:
     case TYPE_HLL:
+    case TYPE_OBJECT:
         *reinterpret_cast<StringVal*>(dst) = _children[child_idx]->get_string_val(ctx, row);
         break;
     case TYPE_DECIMAL:
@@ -373,6 +374,7 @@ bool CaseExpr::any_val_eq(const TypeDescriptor& type, const AnyVal* v1, const An
     case TYPE_CHAR:
     case TYPE_VARCHAR:
     case TYPE_HLL:
+    case TYPE_OBJECT:
         return AnyValUtil::equals(type, *reinterpret_cast<const StringVal*>(v1),
                                   *reinterpret_cast<const StringVal*>(v2));
     case TYPE_DECIMAL: