[Ballista] Support Union in ballista.

[Ted-Jiang]

Which issue does this PR close?

Closes #2032 .

Rationale for this change

Use cargo run --bin datafusion-cli --features ballista -- --host localhost --port 50050
Before

 SELECT 1 as NUMBER union SELECT 1 as NUMBER;
thread 'main' panicked at 'not implemented', ballista/rust/core/src/serde/logical_plan/mod.rs:818:38
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Now

❯ SELECT 1 as NUMBER union SELECT 1 as NUMBER;
+--------+
| number |
+--------+
| 1      |
+--------+
1 row in set. Query took 1.085 seconds.
❯ SELECT 1 as NUMBER union all SELECT 1 as NUMBER;
+--------+
| number |
+--------+
| 1      |
| 1      |
+--------+
2 rows in set. Query took 0.225 seconds.

What changes are included in this PR?

Are there any user-facing changes?

[houqp]

LGTM