Support for custom HTTP post bodies in a handler file or in TICK script

Currently, as described in here: https://docs.influxdata.com/kapacitor/v1.5/event_handlers/post/#options ,
the HTTP post body cannot be customized in a handler file or when using .post() in TICK script.

I think it is sensible to be able to customize this at least in a handler file, too, and not just limit the customization to be available in the kapacitor.conf file only.

Start with the HTTP post handler options described in the linked documentation, then trace how the body is currently read from kapacitor.conf. Inspect the handler-file configuration and the TICK .post() entry point; done means custom post bodies can be supplied through both of those paths, not only the configuration file.

Written by the indexing model from the issue text.