Add code signing step for preview pipeline

[mregen]

Add code signing step if code signing certs and secret access is granted to the pipeline
note: until novotnyllc/NuGetKeyVaultSignTool#77 is fixed, using a private build of the NuGetKeyVaultSignTool

[codecov]

Codecov Report

Merging #1122 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1122      +/-   ##
==========================================
- Coverage   40.01%   40.00%   -0.01%     
==========================================
  Files         271      271              
  Lines      103796   103796              
==========================================
- Hits        41529    41522       -7     
- Misses      62267    62274       +7     

Impacted Files	Coverage Δ
Stack/Opc.Ua.Core/Stack/Tcp/UaSCBinaryChannel.cs	62.85% <0.00%> (-1.64%)	⬇️
...ack/Opc.Ua.Core/Stack/Tcp/ChannelAsyncOperation.cs	59.79% <0.00%> (-1.04%)	⬇️
Stack/Opc.Ua.Core/Stack/Tcp/TcpMessageSocket.cs	70.06% <0.00%> (-0.33%)	⬇️
Libraries/Opc.Ua.Client/Session.cs	22.97% <0.00%> (-0.16%)	⬇️
Stack/Opc.Ua.Core/Types/Utils/Utils.cs	52.30% <0.00%> (+0.09%)	⬆️
...Security/Certificates/DirectoryCertificateStore.cs	85.09% <0.00%> (+0.33%)	⬆️

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