perf: Stop performing deep copies when CometScan source is an exchange

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Because our current Parquet decoding logic reuses mutable buffers, we have to be careful to perform a deep copy before calling operators that could cache data. We do this by wrapping ScanExec in a CopyExec.

However, we use ScanExec both for reading from Parquet scans and also for reading from exchanges (broadcast and shuffle). We only need to perform deep copies in the Parquet case.

Here are the before and after changes:

Before

CopyExec [UnpackOrDeepCopy], metrics=[output_rows=510412, elapsed_compute=432.747µs]
  ScanExec: source=[ShuffleQueryStage (unknown), Statistics(sizeInBytes=223.2 MiB, ...

After

CopyExec [UnpackOrClone], metrics=[output_rows=510412, elapsed_compute=9.006µs]
  ScanExec: source=[ShuffleQueryStage (unknown), Statistics(sizeInBytes=223.2 MiB, ...

What changes are included in this PR?

Use CopyMode::UnpackOrClone instead of CopyMode::UnpackOrDeepCopy when wrapping a CometScan that is reading from an exchange.

How are these changes tested?

Manually.