Let Frontend Pass Physical Plan to ComputingUnit

[Xiao-zhen-Liu]

In the current Texera architecture (shown below), we have two compliers, one being the WorkflowCompilingService, the other WorkflowCompiler in the ComputingUnit.

In the codebase they are mostly duplicated code. We should get rid of WorkflowCompiler eventually to avoid this duplication.

The main blocker currently is that WorkflowCompiler is a part of the execution engine (shown below), which accepts a logical plan from the frontend, and relies on the WorkflowCompiler to produce a physical plan.

If we allow the frontend to pass a physical plan to ComputingUnit instead, We can remove WorkflowCompiler and keep only WorkflowCompilingService.