pw_i2c: Adds a mocked initiator implementation

This is useful as a reusable mock when writing drivers. Specifically
this allows a driver developer to specify a set of mocked i2c
transactions. This does not require physical hardware and is ideal for
unit testing a driver.

Change-Id: I4e0f13450f84feeefeef534b6c46192462c18a8b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/37920
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>