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:
#1409 (merged)

Code PRs:
To be listed once opened

[zhangyanzhao]

duplicated with 1409 item, close this one.