Add Fan interface for SMART devices

[sdb9696]

Enables the Fan interface for devices supporting that component. Currently the only device with a fan is the ks240 which implements it as a child device. This PR adds a method get_module to search the child device for modules if it is a WallSwitch device type.

[codecov]

Codecov Report

Attention: Patch coverage is 88.09524% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 89.23%. Comparing base (7db989e) to head (eceb954).

Files	Patch %	Lines
kasa/smart/smartdevice.py	80.76%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #873      +/-   ##
==========================================
- Coverage   89.27%   89.23%   -0.04%     
==========================================
  Files          67       68       +1     
  Lines        4819     4851      +32     
  Branches     1186     1201      +15     
==========================================
+ Hits         4302     4329      +27     
- Misses        427      429       +2     
- Partials       90       93       +3     

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[rytilahti]

A couple of more nits, but I think this is otherwise ready-to-go 👍

[rytilahti]

Feel free to add a description and merge this 👍