Enable explain query in Ballista.

[Ted-Jiang]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Now

explain select 1 as 'o_orderkey'  union all   select o_orderkey from orders order by o_orderkey limit 5;

[2022-03-26T05:17:46Z ERROR ballista_core::execution_plans::distributed_query] 
Job y4JTvKw failed: Job y4JTvKw failed due to General error: Could not save stage plan: Not implemented: PhysicalExtensionCodec is not provided
Execution("Job y4JTvKw failed: Job y4JTvKw failed due to General error: 
Could not save stage plan: Not implemented: PhysicalExtensionCodec is not provided")

Describe the solution you'd like

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[doki23]

I'd like to take this issue if no one's working on it.

[Ted-Jiang]

I'd like to take this issue if no one's working on it.

Thanks @doki23 ! ❤️

[doki23]

Hmm, I need some help. I have 2 solutions:

1. Generate a stage, and submit it.
2. Ballista scheduler executes the query itself.

The first one needs small changes but leads to unnecessary scheduler workload.
The second doesn't bring scheduler workload, but may need new api for scheduler like execute_explain (I'm not very sure).
How do you think? @Ted-Jiang @yjshen Or is there another way?