Fix XML Encoding and decoding of Matrix Element in Variant to conform to spec

[romanett]

Proposed changes

A Variant containing a multi dimensional array needs to be encoded with a specified syntax.
The XML Encoder / Decoder of the stack did only follow this Spec for the ReadArray / WriteArray implementation but not ReadVariant/WriteVariant

https://reference.opcfoundation.org/Core/Part6/v105/docs/5.3.1.17

This PR fixes JsonEncoder & Decoder & introduces a test case.

#3045

Related Issues

• Fixes XML Encoding of Matrix is Incorrect #3045

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

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

Attention: Patch coverage is 84.37500% with 5 lines in your changes missing coverage. Please review.

Project coverage is 57.03%. Comparing base (95d162e) to head (3f51a09).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
Stack/Opc.Ua.Core/Types/Encoders/XmlDecoder.cs	83.87%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3053      +/-   ##
==========================================
+ Coverage   57.00%   57.03%   +0.03%     
==========================================
  Files         357      357              
  Lines       68916    68951      +35     
  Branches    14154    14156       +2     
==========================================
+ Hits        39285    39328      +43     
- Misses      25385    25388       +3     
+ Partials     4246     4235      -11     

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