[extint] Adds external interrupt support for SAM

[henrikssn]

Solves #435

Let me know if you have any feedback on the approach, I am green-fielding this area and it would be cool if we could eventually match the interrupt implementations for the other platforms to this.

The end goal would be to be able to compile the example added in this PR for any platform that supports interrupts.

[salkinium]

An advanced alternative to allocating the function at runtime would be to place the function addresses into a linker section (analog to how MODM_HARDWARE_INIT works) and run through that list in the interrupt. That would delegate memory allocation for this to the linker, but it would not allow for runtime addition/removal, but you could allocate RAM space (based on the list size) for a bit vector to enable/disable each handler separately at runtime.

However, you could probably get 90% there with an lbuild configurable handler list in RAM, so… 🤷‍♂️

[henrikssn]

Not sure what is up with CI, I hope it is not my fault :)

[salkinium]

I'm ok to merge this if you are (and I understand what's wrong in the CI).

[henrikssn]

SGTM, ready to merge.