Support UseAlternativeLoadPath for native DLL lookup

Some build tooling has more esoteric loading of dependencies (Cake for example), and requires custom path lookup, particularly when dealing with native dependencies, as this converter has on `Microsoft.DiaSymReader.Native`.

`SymUnmanagedWriterFactory.CreateWriter` supports this via `SymUnmanagedWriterCreationOptions.UseAlternativeLoadPath`, but the current converter code doesn't offer this:
https://github.com/dotnet/symreader-converter/blob/b52ea77026652e6a29f6074de697d9eb8148a071/src/Microsoft.DiaSymReader.Converter/PdbConverter.cs#L169

Currently I have to clone the code to add this option to get the native DLL loading working. It would be extremely useful to have this available, either just included or as an option.