fix a bug of const union query like `select null union select null`, this because the type of SlotDescriptor when clause is `select null` is null ,this will cause BE core dump, and FE find wrong cast function.
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.
