Add device_id property to Device class

[starkillerOG]

This PR adds new device_id property to Device class.
The device_id is part of the handshake reply and thus available also on devices with unknown tokens (i.e., it can be useful for use cases where the token is extract from the cloud or other external sources).

This is also needed for push server implementation: #1288

[starkillerOG]

@rytilahti could you merge this?

[codecov-commenter]

Codecov Report

Merging #1384 (b929d79) into master (7269b15) will decrease coverage by 0.01%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #1384      +/-   ##
==========================================
- Coverage   84.17%   84.15%   -0.02%     
==========================================
  Files         133      133              
  Lines       13319    13324       +5     
  Branches     1483     1484       +1     
==========================================
+ Hits        11211    11213       +2     
- Misses       1897     1900       +3     
  Partials      211      211              

Impacted Files	Coverage Δ
miio/device.py	53.89% <40.00%> (-0.47%)	⬇️

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[starkillerOG]

@rytilahti processed the feedback

[starkillerOG]

@rytilahti could you merge?

[rytilahti]

This will cause I/O if the device hasn't been initialize prior trying to access the property, but I think the pros outweigh the cons. If needed, this can be revisited in some later point.

Thanks @starkillerOG, this is good to go as soon as some tests are added to check the both code paths! 👍

[starkillerOG]

@rytilahti I just added a test, could you check if that is what you ment.