Add a --native-only dotnet-symbol that only downloads native binaries

[mikem8361]

This will allow an improved xplat/lldb core dump work flow. All the managed PDBs are not downloaded (10 to 15 minutes for a simple webapp). SOS does a good job at download the PDBs it needs on demand.

The --host-only works if the user doesn't care about native symbols and modules, but not for 5.x (master) runtime core dumps. lldb works a lot better with all the native runtime modules (mainly libcoreclr.so) present next to the core dump when lldb loads the dump.