[Events] Add an file event wakeup method with socketpair

*Description*:

The goal is to create a premitive in `source/common/event` that wakes up (schedules) an event using a socketpair instead of `scheduleCallbackCurrentIteration`. The reason for that is that `scheduleCallbackCurrentIteration` is using ` event_active(&raw_event_, EV_TIMEOUT, 0);` which takes a lock.

See comment in: https://github.com/envoyproxy/envoy/pull/13954#discussion_r533796896

This mechanism could be applied to:
1. `Win32` DirectoryWatcher
2. Replace the wakeup mechanism in `dispatcher_impl.cc` -> `void DispatcherImpl::post(std::function callback)`

For `Win32` it is good to also have #10871 if we are going to build on top of socketpairs