[XLANG] add union to xlang type system

[chaokunyang]

Feature Request

add union to xlang type system

Is your feature request related to a problem? Please describe

No response

Describe the solution you'd like

union is used wildly in many languages:

• rust: tagged enum
• c++: variant

And other systems all has union:

• protobuf has oneof
• FlatBuffers has union
• Arrow has sparse/dense union array

We can add a UNION type too. the protocol can be:

ref meta | UNION type id | union index  | value |

Tracking issue:

• [C++] Support C++ variant serialization #3025
• Use union type id for tagged enum serialization in rust #3028
• Implement union type in python #3029
• Implement union type in java #3030
• Implement union type in go #3031

Describe alternatives you've considered

No response

Additional context