Benchmark manual samples and iterations count

Currently, Catch2 Benchmark automatically compute the required iterations count for a benchmark to be accurate, and the sample count seems to be fixed to 100.

This is great. But in some cases, it would be very useful to be able to set this manually, especially the sample count. This is possible in Celero, and it would be great to have it too in Catch2.

Start by locating Catch2's benchmark implementation and existing benchmark tests, then trace how sample and iteration counts are currently selected. Done means exposing a documented way to configure those counts, especially the sample count, with tests covering the new behavior.

Written by the indexing model from the issue text.