Finalize callback list implementation, and migrate wifi events to use it.

### Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [x] I have tested that the issue is present in current master branch (aka latest git).
- [x] I have searched the issue tracker for a similar issue.
- [x] If there is a stack dump, I have decoded it.
- [x] I have filled out all fields below.

#### Platform

- Hardware: all
- Core Version: 2.5.0
- Development Env: all
- Operating System: all

### Settings in IDE

- Module: all

### Problem Description

After some discussion, @hreintke pulled out the wifi callback event list implementation into a proposed standalone implementation. That implementation needs to be tested, covered by tests, examples, and the wifi event system needs to be migrated to use it. Once that is done, other callback systems could potentially make use of it.

Ref: #5710