periph/gpio: support for extension API (part 2)

[ZetaR60]

Contribution description

This is an implementation of GPIO redirection outlined by my proposal in #9582 (items 5-6) for the periph/gpio interface. In short, it allows other drivers (such as GPIO expanders) to be addressed using gpio_t so that all existent code can easily gain support through those drivers. For non-implementation detail and discussion, please see #9582

At @kYc0o 's suggestion, I am breaking the implementation of #9582 / #9690 into several pieces to make it easier to review. This PR only contains the API redirection code. For the intercept code (part 1) see #9860

Dependencies

This depends on #9860

Testing procedure

A test case is provided that implements a soft-driver for the GPIO extension interface. The test case uses this driver to confirm that interception and redirection of the API call are working properly. This has been tested and is working properly on mega-xplained.

Issues/PRs references

Partially replaces and closes #9190
Implements items 5-6 (redirection) of my proposal in #9582
For implementation of items 1-4 (interception) see #9860
Part of work tracked by #9690

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.