Support client activities on all target platforms

[mregen]

Proposed changes

• Add the system diagnostics library for unsupported target platforms
• Integrate in clientfixture by adding IDisposable to support to enable/disable the activity tracing
• By default only use it on .NET 6 and later

Types of changes

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• 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

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• 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

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

Merging #2256 (f53a1b7) into master (cfb3ac4) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2256      +/-   ##
==========================================
- Coverage   58.95%   58.85%   -0.11%     
==========================================
  Files         330      330              
  Lines       63198    63199       +1     
==========================================
- Hits        37257    37194      -63     
- Misses      25941    26005      +64     

Files Changed	Coverage Δ
Libraries/Opc.Ua.Client/TraceableSessionFactory.cs	30.00% <ø> (ø)
Libraries/Opc.Ua.Client/TraceableSession.cs	51.14% <100.00%> (ø)
Stack/Opc.Ua.Core/Types/BuiltIn/DiagnosticInfo.cs	73.21% <100.00%> (+0.16%)	⬆️

... and 13 files with indirect coverage changes