[C++][Parquet] Always transfer to executor in Parquet AsyncGenerator

[asfimport]

After ARROW-12560 and ARROW-11843 land, we should force Parquet decoding work to be transferred onto the CPU executor to ensure we don't block an I/O thread.

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

Related issues:

• [C++] Add asynchronous read to parquet::arrow::FileReader (Dependent)
• [C++] Investigate utilizing aggressive thread task creation when adding callback to finished future (Dependent)
• [C++] Implement OptionalParallelForAsync (duplicates)

PRs and other links:

• GitHub PR#10482

_{Note: This issue was originally created as ARROW-12916. Please see the migration documentation for further details.}