[➕ Feature]: Allow custom JSON input for Grafana OnCall Provider

[ameijboom]

Is your feature request related to a problem? Please describe. With the current version of the Grafana OnCall Provider, there is a limited set of information we can send with our alerts. Grafana supports custom templating and accepts key/value pair in the body of requests sent to their OnCall API yet we're currently limited to only a select few parameters.

Describe the solution you'd like Simply adding a new parameter (such as body, payload or json) should be enough. If the parameter is passed to the provider, we send the value instead of creating a json object based on other parameters ourselves.