.natvis for external modules is not supported

Currently UBT supports linking in .natvis files when modules are compiled, this should be extended to pull in .natvis files from External modules that are linked into a module.

Instead of scanning the External module folder for .natvis files, these should be added to a list manually in the External Build.cs same as how we add .lib files and then propagated to any modules\binaries that reference the External module.

1. Create an External module.
2. Note there is no way to add .natvis files to the configuration to be linked in to referencing modules.