Enable email notifier to send emails for health check failures

[prakhar10]

Description

This change enables to send email notifications for healthcheck failures on backend trino clusters.

New fields added in the NotifierConfiguration.java class: enabled - To enable or disable sending emails for healthcheck failures gatewayInstance - To specify gateway instance name in case there are multiple instances. This will be displayed in the email subject so that one can know in which instance the cluster is unhealthy. customContent - To provide custom email body in case the default content is not very helpful.

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