Renew Cognito credentials on MQTT reconnection

[mochipon]

Due to the websocket connection quota in AWS IoT, the websocket connection will be closed after 24 hours. The problem is the temporary credential from Cogninto has been out dated at that moment so we cannot reconnect then.

This PR aims to get new credentials from Cognito when the MQTT session is disconnected. Note that the callback (onOffline) does not block the process. It is possible that reconnection has already started with an old credential. The SDK will continue to retry, so it will eventually reconnect successfully once the new credentials are obtained.

fixes #36

[codecov]

Codecov Report

Merging #37 (c21aea6) into main (92833dc) will decrease coverage by 1.44%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
- Coverage   95.88%   94.44%   -1.45%     
==========================================
  Files           8        8              
  Lines         389      396       +7     
==========================================
+ Hits          373      374       +1     
- Misses         16       22       +6     

Impacted Files	Coverage Δ
pysesame3/chsesame2.py	80.20% <14.28%> (-5.19%)	⬇️

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