Add support for pairing devices

[rytilahti]

This implements "child_quick_setup" that allows pairing and unpairing devices with a hub.
Three new cli commands are added to make the process smooth:

• kasa child list lists current children and their device ids
• kasa child pair searches for unpaired devices and performs pairing after something is discovered. The search ends immediately after finding the first device that can be paired.
• kasa child unpair <device_id> allows unpairing an existing device.

TBD:

• Documentation for hubs in general
• Expose pairing as an action when Implement action feature #849 is done
• Tests

[codecov]

Codecov Report

Attention: Patch coverage is 42.02899% with 40 lines in your changes are missing coverage. Please review.

Project coverage is 88.46%. Comparing base (c5d65b6) to head (b6d5eb1).

❗ Current head b6d5eb1 differs from pull request most recent head 7f302aa. Consider uploading reports for the commit 7f302aa to get more accurate results

Files	Patch %	Lines
kasa/smart/modules/childsetup.py	37.50%	24 Missing and 1 partial ⚠️
kasa/cli.py	46.42%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #859      +/-   ##
==========================================
- Coverage   90.48%   88.46%   -2.02%     
==========================================
  Files          70       68       -2     
  Lines        4937     4732     -205     
  Branches     1218     1165      -53     
==========================================
- Hits         4467     4186     -281     
- Misses        380      459      +79     
+ Partials       90       87       -3     

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[rytilahti]

Just to avoid forgetting about it, pairing/removing a device will require us to perform the initial update again in the main device to initialize the newly added devices. Currently the subsequent updates will receive a new device, but the children is not updated accordingly.

For homeassistant UX, we will also need a way to re-trigger platform initialization for the new device.