Add object type

[kangkaisen]

For #1610

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.

[kangkaisen]

Update.
Use bitmap type in FE and convert to object type in Thrift.

[imay]

Should update correspond document.

[kangkaisen]

1 Object type support v1
2 Change the bitmap_union type to bitmap
3 Add check for bitmap_count function
4 Update the doc

[imay]

LGTM