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ARROW-17287: [C++] Create scan node that doesn't rely on the merged generator (#13782)

**Primary Goal:** Create a scanner that "cancels" properly.  In other words, when the scan node is marked finished then all scan-related thread tasks will be finished.  This is different than the current model where I/O tasks are allowed to keep parts of the scan alive via captures of shared_ptr state.

**Secondary Goal:** Remove our dependency on the merged generator and make the scanner more accessible.  The merged generator is complicated and does not support cancellation, and it currently only understood by a very small set of people.

**Secondary Goal:** Add interfaces for schema evolution.  This wasn't originally a goal but arose from my attempt to codify and normalize what we are currently doing.  These interfaces should eventually allow for things like filling a missing field with a default value or using the parquet column id for field resolution.

Performance isn't a goal for this rework but ideally this should not degrade performance.

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>