Add headers support to grafana_contact_point webhook

[philthynz]

Feature Request

Grafana v12 is now released and supports custom headers in webhooks. PR: grafana/grafana#103818

Feature request to add headers to webhook in grafana_contact_point resource.

Example:

resource "grafana_contact_point" "contact_point_b9665ac0e4fed046" {
  name = "MyWebHook"

  webhook {
    url = "https://webhook_address"
    headers = {
      x-sn-apikey = "apikey"
    }
  }
}

[loic-vial]

It would be nice to have a support for the new "payload" field too :)

[JacobsonMT]

Completed in #2331