Enable support for sending alerts on Pagerduty

[pranay01]

Currently SigNoz only supports sending alerts on Slack channels. Pagerduty is one of the most popular incidence management platform. Support form sending alerts from SigNoz to PagerDuty would enable such users to manage their alerts more effectively.

This would require

1. Ability to create PagerDuty as a new notification channel - Should be relatively simple to do as Alertmanager already supports pagerduty
2. Ability to configure which alerts is sent to which channels. This would require creating a mapper between alert types and the notification channels

[pranay01]

this is fixed now. Users can specify which channels they want the alert to go to when specifying alert rules