pw_interrupt fake backend

The fake backend is meant for use in unit tests when run on the host.

The fake backend implementation can be found in pw_interrupt/fake_context.cc, and also has a corresponding header pw_interrupt/fake_context.h for use by the test.

With the fake backend, the InInterruptContext() call will by default return false.

The fake backend function SetInInterruptContextCallback() can be used to set a lambda function which allows the test to return an arbitrary value.

See the example unit test in pw_interrupt/examples/context_usage_test.cc.