Add support for certificate based MQTT authentication

[jnsjll]

Proposed changes

Preserve a client certificate's private key in MQTT.net TLS options to enable certificate based authentication.

Related Issues

• Fixes [PubSub] Certificate based MQTT authentication not possible #3226

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 66.27%. Comparing base (d25caff) to head (fb3fcc9).
⚠️ Report is 140 commits behind head on master.

Files with missing lines	Patch %	Lines
...es/Opc.Ua.PubSub/Transport/MqttPubSubConnection.cs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3610       +/-   ##
===========================================
+ Coverage   51.86%   66.27%   +14.40%     
===========================================
  Files         370      459       +89     
  Lines       78618    97819    +19201     
  Branches    13650    16382     +2732     
===========================================
+ Hits        40779    64827    +24048     
+ Misses      33705    27926     -5779     
- Partials     4134     5066      +932     

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[CLAassistant]

All committers have signed the CLA.