* Add config option to leverage all cores for sabre

By default when running sabre in parallel we use a fixed number of
threads (depending on optimization level). This was a tradeoff made for
having deterministic results across multiple systems with a fixed seed
set. However when running qiskit on systems with a lot of CPUs
available we're leaving potential performance on the table by not using
all the available cores. This new flag lets users opt-in to running
sabre with n trials for n CPUs to potentially get better output results
from the transpiler, with minimal to no runtime overhead, at the cost of
the results not necessarily being reproducible when run on a different
computer.

* Apply suggestions from code review

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* Rework logic to use the default if larger than CPU_COUNT

This commit refactors the logic added in the previous commit to a single
helper function. This reduces the code duplication and makes it easier
to work with. While doing this the logic has been updated so that when
the flag is set and the default number of trials is larger than the
CPU_COUNT we use the default. This means the logic when the flag is set
is to run `max(default_trials, CPU_COUNT)` which should better match
user expectations around the flag.

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Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>
(cherry picked from commit f8ac2ad)