Disable validateReferences in DLLs meta files #140

From Unity version 2020.1 and on, Unity has added a validation step for strongly named assemblies and throws an error when the version doesn't match exactly, which is quite common in NuGet packages. This makes this package completely useless in newer version.
Adding "validateReferences:0" to the DLL meta file converts the error into a warning, making it usable again. This should be added in the UnityMetaFileGenerator.targets:65 file.

It could also be nice to add "isExplicitlyReferenced: 1" to force to reference the exact DLLs from each AssemblyDefinition, instead of referencing all everywhere. Or a setting to make this optional could be nice.