Expand the ResilienceEvent with the Severity

[martintmk]

This allows us to report Informational events and use the common-infra.

Changes:

• Add/Modify methods on ResilienceStrategyTelemetry to accept the serverity.
• Make sure that TelemetryResilienceStrategy strategy reports unhealthy status only for events with high severity.
• Consider whether to introduce new enum or re-use some existing one.