Implement low power mode api

### 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: latest git today
- Development Env: All
- Operating System: All

### Problem Description

PR #6989 provides a low power usage reference example, and it has been merged. Per [@d-a-v 's comment](https://github.com/esp8266/Arduino/issues/6642#issuecomment-575822115), it should be investigated how to unify the current experimental pseudo modes with the reference code in the example, and how to provide a consistent api to the users. Once that api is available, the example in #6989 should be migrated to make use of that api without changing the current behavior of the tests in that example sketch.

### [MCVE](https://stackoverflow.com/help/mcve) Sketch

See the example merged in #6989.