Events/Alarms Framework

[jeff-yin]

Assign this to @bhaveshdell

This initiative is to introduce an events/alarms framework that persists events/alarms in the DB for persistence and management/monitoring services. It will also provide a gNMI/REST openconfig interface for subscriptions and GET operations.

HLD: https://github.com/sonic-net/SONiC/pull/1409/ (merged)

Code PRs: To be listed once opened