Add connection_error handler

[eddbbt]

Add :connection_error exception handler when executing a trigger policy, then try to re-enqueue as 408 (timeout) error

resolves #936

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 62.23%. Comparing base (20233b7) to head (37fffa6).
Report is 3 commits behind head on release-1.1.

❗ Current head 37fffa6 differs from pull request most recent head daa5cd7

Please upload reports for the commit daa5cd7 to get more accurate results.

Files	Patch %	Lines
...engine/lib/astarte_trigger_engine/policy/policy.ex	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.1     #943      +/-   ##
===============================================
- Coverage        62.25%   62.23%   -0.02%     
===============================================
  Files              193      193              
  Lines             4726     4727       +1     
===============================================
  Hits              2942     2942              
- Misses            1784     1785       +1     

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