[C++] Simplify ExecPlan's C++ interface

[asfimport]

In recent work documenting the ExecPlan interface (ARROW-15091) and in trying to make a simple example using it (#11799) we are seeing some rough edges. This parent issue will track a number of subtasks that should make the C++ API easier to work with.

Reporter: Weston Pace / @westonpace

Subtasks:

• [C++] Easy options to create a source node from a table
• [C++] Simplify creating a scan node
• [C++] Create a ToTable sink node
• [C++] Add ExecNode to arrow/compute/api.h
• [C++] The write node options shouldn't require a schema
• [C++] Update ExecPlan example code and documentation with new options

Related issues:

• [C++] Create "engine" module for the query engine (depends upon)

_{Note: This issue was originally created as ARROW-15257. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.