Webhook Action UI - allow context variables to be added to body

[peterschretlen]

Related to #58895

When attaching a Webhook action to an alert, the body parameter is a string. However it's not evident that context variables + mustache can be used on these parameters.

To make it clearer and assist the user, we should add the context variable picker to all fields, like we do for the message field in a slack action.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[peterschretlen]

For 7.7 it might be worth assuming the body will be json and using the same UI as the index action UI for specifying a document. We can extend to add support for other content types later?

[alexfrancoeur]

🎉