Feature: provide a way that connectors can run seperatorly

[jaredzhou]

Feature description

find this repo lately, really like it . here is something that i find
when i try to run the conduit-connector-postgres, it report

This binary is a plugin. These are not meant to be executed directly.
Please execute the program that consumes these plugins, which will
load any plugins automatically
exit status 1

since plugin and server is communicating with grpc, it's easy to allow plugin run independently, at least it can easy for developer to debug plugin

[jaredzhou]

i seem like a go-plugin problem