Provide a way to specify surfacer config independently

[manugarg]

In an enterprise setup, where you might be running probes in multi-tenant mode, you might want surfacers to be controlled centrally, while rest of the config is provided by tenants.

We can do something like: if --surfacer_config_file flag is given, use that config to get surfacers. We can process this config file pretty much the same way as the regular config file, but just ensure that parsed config contains only surfacers in it.