Add Header Support for Custom Events monitoring config

Hi Cycle team,

I noticed two new endpoints in the environment monitoring config for metrics and events. For custom events, it looks like the only option available right now is providing a destination URL.

We use Atlassian Opsgenie, which uses a global destination url and requires sending an API key to be included in the request header for authorization. Example:

curl -X POST https://api.opsgenie.com/v2/alerts \
  -H "Authorization: GenieKey API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "message": "CPU usage critical",
    "tags": ["cpu", "production"]
  }'

It would be helpful to have an option to include an API key (or custom headers) as part of the configuration, so it can be sent along with the request.

This enhancement would enable direct integration with services like Opsgenie and other systems that require header-based authentication, improving flexibility and reducing the need for intermediary solutions.