Make it easier to build custom scheduler and executor processes that extend Ballista

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
For users wanting to extend Ballista with custom object stores, table providers, and udfs, the easiest approach is to build a new project that uses Ballista as library and provides its own scheduler and executor processes that register the custom components.

This currently involved copy and pasting a lot of code from Ballista's main methods.

Describe the solution you'd like
Refactor the main methods to be minimal and call into Ballista as a library to make this more reusable.

Describe alternatives you've considered
None

Additional context
None

[milenkovicm]

this is mostly done, schedulers and executors can be created by users, there is even examples how to do it.
can we close this issue @andygrove ?