dotnet-svcutil: optimize the referenced wcf package version and add test.

[imcarolwang]

Addresses #5499.

Description:

1. Correctly resolve netstandard2.0 if the TFM moniker string was passed from VS (WCF CS Tool)
2. Create clear table mapping between .NET target version and wcf package version, for the various wcf releases which support netstandard2.0, keep the latest version 4.10.* only; update CloseAsync() generation logic to generate it for TFM less than 6.0, and update test baselines
3. Add only the common wcf references for version 8.0 packages, update corresponding test baselines
4. Improve wcf assembly reference condition for .NET framework target (use $(TargetFramework.StartsWith('net4') instead of getting the first .NET target and check for equality, this will fix bug when there're multiple .NET framework 4.x targets)

[HongGit]

/azp run

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