event_timers: Add module

This CL adds a library that can be used to manage multiple timers set by
pubsub events. Timers are identified by a token, and only the most
recently set timer for each token will be triggered.

Change-Id: I90ca2c5878aa8f2fa6343a5ff364e1b774224e16
Reviewed-on: https://pigweed-internal-review.git.corp.google.com/c/pigweed/showcase/rp2/+/73136
Reviewed-by: Taylor Cramer <cramertj@google.com>
Commit-Queue: Aaron Green <aarongreen@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>