Support for message priorities

[abompard]

This change allows users to set the priority on messages. Reference : https://www.rabbitmq.com/priority.html

[codecov-commenter]

Codecov Report

Base: 88.67% // Head: 88.67% // No change to project coverage 👍

Coverage data is based on head (9a94dcf) compared to base (9a94dcf).
Patch has no changes to coverable lines.

❗ Current head 9a94dcf differs from pull request most recent head 1d65a1c. Consider uploading reports for the commit 1d65a1c to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #275   +/-   ##
========================================
  Coverage    88.67%   88.67%           
========================================
  Files           13       13           
  Lines         1183     1183           
  Branches       174      174           
========================================
  Hits          1049     1049           
  Misses         121      121           
  Partials        13       13           

Flag	Coverage Δ
unittests	88.58% <0.00%> (ø)

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