Add support for connecting as named user

[OliverWoolland]

Summary

Adds optional authentication support by allowing username and password to be set via CLI arguments.

This should enable connections to brokers which disallow anonymous connections or if authentication is expected.

Defaults remain unchanged (guest/guest), so existing usage should be unaffected.

Details

• Adds --mqtt-username and --mqtt-password CLI args to pytest-mqtt
• Updates the fixtures / clients to accept credentials
• Documents new arguments in README.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.08%. Comparing base (ab524fd) to head (0e29dba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   95.90%   96.08%   +0.18%     
==========================================
  Files           5        5              
  Lines         171      179       +8     
==========================================
+ Hits          164      172       +8     
  Misses          7        7              

Flag	Coverage Δ
unittests	96.08% <100.00%> (+0.18%)	⬆️

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