[FEATURE] Enhance custom webhooks to support custom message formats for request bodies

[AWSHurneyt]

Is your feature request related to a problem?
Currently, custom webhook channels directly use the message string as the request body when sending a notification. However, some webhooks require other formats for the request body. E.g., Google chat require a { text: "message content" } format (documentation link) similar to Slack webhooks.

What solution would you like?
Custom webhooks should be able to support custom message formats to accommodate more webhook types.

[fbarbeira]

Mattermost is also affected with this issue:

[2024-02-26T08:12:41,734][ERROR][o.o.n.c.t.WebhookDestinationTransport] [os-manager] Exception sending webhook message b8pC5I0BK_x004ISOKmL: org.opensearch.notifications.spi.model.MessageContent@5f7e7f1
java.io.IOException: Failed: {"id":"model.incoming_hook.parse_data.app_error","message":"Unable to parse incoming data.","detailed_error":"invalid character 'T' looking for beginning of value","request_id":"yr5iwditu3rnurdh8yrjp3no3o","status_code":400}