(dagit-reload-1/n) Add GetServerId gRPC service method
Summary: This diff adds a GetServerId that exposes a unique UUID associated with each server. This is used to help track gRPC servers updating.
Test Plan: unit
Reviewers: alangenfeld, dgibson, prha, max
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D5105