pdm: add PDM library support

[maidnl]

The purpose of this PR is to add support for PDM.
At the present this version of the PDM library only supports Nano 33 BLE Arduino board.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}