(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