GRPC Plugin support for discovery agent

Earlier, third party plugins could send telemetry to discovery process over a unix socket. This was disabled a while ago as there is no clean way to implement authentication for plugins.

Refactor this to be a GRPC interface so other scanning tools (e.g secret scanner and more) can be written in any language and communicate with agent to share telemetry and advanced detections. Use auth.

For exmaple:

secret scanner/other plugins -----grpc----> agent -------------> console