User callbacks to timer peripheral

[stephenhensley]

This was useful in the case where there were low priority "background" tasks that need to happen at a regular interval (UI updating, Eventqueue handling, etc.), but actual background tasks that only function with blocking I/O for now (i.e. QSPI writes, and USB Host background process).

Without having the UI, etc. in an IRQ they would stutter/stop/glitch during the QSPI writes/USB processing.

In the rare case where there's a decent amount of processing happening in these callbacks, the Systick was getting missed. So I bumped the NVIC priority of systick to have one position below it.

I'd like to add a priority setting to these callbacks so the user can set it up at the desired level, but for now these callbacks have the lowest priority in the whole system.

[github-actions[bot]]

Unit Test Results

    1 files  ±0    16 suites  ±0   0s :stopwatch: ±0s 147 tests ±0  147 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 5d8987bc. ± Comparison against base commit e12fb04e.

:recycle: This comment has been updated with latest results.

[stephenhensley]

TODO:

• [x] Add IRQ enable, etc. for remaining peripherals
• [ ] ~Add configurable IRQ Priority (0-15) to match NVIC config.~ (moved to todo tag for now)
• [x] add enable_irq bool to config to avoid unnecessary IRQs if not desired
  • this seems a better fit than having it be based on whether the callback is set or not since the user can change that at will.
• [x] Add doxygen documentation because it's all missing currently