Implement device scanning

[dmulcahey]

fixes: #115

This is VERY WIP at the moment. Just getting some ideas into code to see what things look like. Feel free to drop comments / ideas here as I work through this.

[codecov]

Codecov Report

Attention: Patch coverage is 12.30769% with 57 lines in your changes missing coverage. Please review.

Project coverage is 95.32%. Comparing base (57cc44b) to head (9532c57).

Files with missing lines	Patch %	Lines
zha/zigbee/cluster_handlers/__init__.py	10.20%	44 Missing ⚠️
zha/zigbee/device.py	20.00%	8 Missing ⚠️
zha/zigbee/endpoint.py	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #190      +/-   ##
==========================================
- Coverage   95.89%   95.32%   -0.58%     
==========================================
  Files          61       61              
  Lines        9404     9469      +65     
==========================================
+ Hits         9018     9026       +8     
- Misses        386      443      +57     

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