feat(grafana-oncall): add custom_json parameter for custom payloads

[ningyang1988]

Summary

Adds support for a custom_json parameter in the Grafana OnCall provider's _notify method, as requested in #5316.

Problem

Currently, the Grafana OnCall provider only sends a fixed set of fields (title, message, alert_uid, image_url, state, link_to_upstream_details). Grafana OnCall supports custom templating and arbitrary key/value pairs in webhook payloads, but there was no way to pass custom fields through Keep.

Solution

• Added a custom_json parameter (type: dict | str | None) to _notify
• When custom_json is provided:
  • If it's a string, it's parsed as JSON
  • If it's a dict, it's used directly
  • The custom payload is sent as-is, bypassing default field mapping
• When custom_json is None (default), the existing behavior is unchanged

Changes

• keep/providers/grafana_oncall_provider/grafana_oncall_provider.py: Add custom_json parameter and payload routing logic
• docs/providers/documentation/grafana_oncall-provider.mdx: Add "Custom JSON Payload" section with examples
• docs/snippets/providers/grafana_oncall-snippet-autogenerated.mdx: Add custom_json to parameter list
• tests/providers/grafana_oncall_provider/test_grafana_oncall_custom_json.py: Unit tests covering all code paths

Testing

All 5 unit tests pass:

• Default payload (no custom_json)
• custom_json as dict
• custom_json as JSON string
• custom_json=None fallback
• Invalid JSON string error handling

Usage

actions:
    - name: Custom alert
      provider: grafana_oncall
      config: "{{ provider.my_provider_name }}"
      with:
        custom_json:
          alert_uid: "custom-123"
          title: "Custom Alert"
          state: "alerting"
          my_custom_field: "custom value"

Closes #5316

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[ningyang1988]

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