Add an option to generate the suppressions in the linker attribute XML format

[vitek-karas]

Currently the --generate-warning-suppressions will produce the suppressions in C# - as a C# source code using the attribute syntax. This is useful if I want to make the suppressions part of my shipping code (as I include the source file with my project), but it's not useful if I only want to use the suppressions within the build system of my project, but not include the suppressions in the final bits.

One way to do the latter would be to produce the suppressions in the XML format, and only pass them as a command line for the linker - for that invocation of the linker those suppressions would take effect, but they would not get baked into the final build.

See dotnet/runtime#39133 for detailed discussion why this is an issue in the dotnet/runtime repo

As a technical solution, we could change the --generate-warning-suppressions option to take a value (not just true/false) - so basically either --generate-warning-suppressions cs or --generate-warning-suppressions xml.