feat: add custom payload support to Grafana OnCall provider

[DragonBot00]

Summary

Adds an optional payload parameter to the Grafana OnCall provider's _notify method, allowing users to send a fully custom JSON body to the OnCall webhook instead of the default structured payload.

Motivation

The Grafana OnCall webhook accepts arbitrary key-value pairs for custom templating, but the provider was limited to a fixed set of fields (title, message, alert_uid, image_url, state, link_to_upstream_details). This prevented users from leveraging OnCall's full templating capabilities.

Closes #5316

Changes

• Added optional payload: dict | None = None parameter to _notify
• When payload is provided, it is sent directly to the webhook — giving full control over the request body
• When payload is None (default), the existing structured body is used — fully backwards compatible
• Fixed duplicate __init__ definition in the original file (two __init__ methods were defined)
• Cleaned up minor formatting issues

Usage example

- name: notify-oncall
  provider:
    type: oncall
    config: "{{ providers.grafana_oncall }}"
  with:
    payload:
      title: "{{ alert.name }}"
      custom_field: "{{ alert.labels.severity }}"
      runbook_url: "https://wiki.example.com/runbooks/{{ alert.name }}"

Testing

• Existing behaviour (no payload argument) remains unchanged
• When payload is passed, it is forwarded directly without modification

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