Allow ballista_scheduler to be embedded in the process of other applications

[r4ntix]

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

Recently, I am working with Ballista as a distributed query engine in our existing scheduling service systems.

I'm trying to embed ballista_scheduler into our already existing scheduling system due to the following cases:

• We have some internal private implementations of TableProvider that cannot be serialized into ballista_scheduler through the ballista_client using protobuf.
• Need to integrate ballista_scheduler and our existing scheduling system into one process.

I found that although ballista_scheduler has exposed the scheduler_server struct through the pub mod scheduler_server, but it still cannot be integrated and embedded.

Because the state and submit_job in the scheduler_server only are pub(crate):
https://github.com/apache/arrow-ballista/blob/20891ae0a740c03b5a3a909ca033f45d59fcfc83/ballista/scheduler/src/scheduler_server/mod.rs#L59-L66
https://github.com/apache/arrow-ballista/blob/20891ae0a740c03b5a3a909ca033f45d59fcfc83/ballista/scheduler/src/scheduler_server/mod.rs#L150-L167

Describe the solution you'd like
Expose state and submit_job in the scheduler_server to pub.

Describe alternatives you've considered

Additional context

[r4ntix]

@andygrove @yahoNanJing Could you please give some feedback and suggestions?

[thinkharderdev]

@r4ntix What would you need from state? I'm not so sure exposing that as a public interface is a great idea. For the most part the grpc interface has evolved to the point where it is primarily just pushing events into the scheduler's event loop so it wouldn't be major issue to expose those as public methods from SchedulerServer itself.