Add relative working directory to NerdbankGitVersioning attribute

[Sam13]

Adds support for mono repos by allowing usage of different configuration files in subdirectories. See https://github.com/dotnet/Nerdbank.GitVersioning/blob/main/doc/pathFilters.md for documentation about multiple NBGV configuration files in same repository.

Usage:

[NerdbankGitVersioning(WorkingDirectoryRelativePath = @"subdir\different-config\")]
readonly NerdbankGitVersioning Nbgv;

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[Sam13]

@matkoch I don't think I'm responsible for all those Quodana errors during PR build :-|