Alert manager SNS receiver have option to publish message with json format

A lot of use-case for SNS is to use lambda to post process message and publish to slack etc..
It will be helpful if we can have an option to enable the web-hook similar json format for SNS messages so that post-processing will be easier
webhook: https://prometheus.io/docs/alerting/latest/configuration/#webhook_config

It will also helpful to keep the customizable template like non-webhook receivers so that if user is using SNS supported direct destination as email and phone text messages, users can still customized something formatted with way they want without post-processing.

The best way to do it might be we have a flag in the sns_configs to enable/disable json format to have users define which way they like(json or template formatted)

• Alertmanager configuration file:

alertmanager_config: |
  global:
    resolve_timeout: 5s
  route:
    receiver: example-sns
    group_interval: 1m
    group_by: [cluster, alertname]
  receivers:
    - name: example-sns
      sns_configs:
      - api_url: https://sns.us-west-2.amazonaws.com
        topic_arn: arn:aws:sns:us-west-2:xxx:My-Topic
        attributes:
          key: severity
          value: SEV2
  inhibit_rules:
    - source_match:
        severity: 'critical'
      target_match:
        severity: 'warning'
      equal: ['low_connected_users']

Start by locating the sns_configs entry in Alertmanager’s configuration and compare the expected SNS payload with the webhook_config documentation linked in the issue. Review the existing SNS receiver behavior and its customizable template support; done means the JSON-versus-template choice is defined for the shown configuration and covered by tests.