Allow POST in HTTPJSONPath lookup table data adapter

## Expected Behavior
The `HTTPJSONPathDataAdapter` lookup table data adapter should support the POST method and allow specifying a body template that gets evaluated with the key that is being looked up.

## Current Behavior
The current implementation of the `HTTPJSONPathDataAdapter` data adapter only supports the GET method to retrieve data from the remote location.
For third party systems that model their searches as POSTs, such as MISP or even Graylog itself, only support GET is not enough.

## Possible Solution
Add a radio button or drop down for the HTTP method (radio if all we want to support is GET and POST, I think the others don't make sense right now).
Allow pasting a body template for POST and evaluate it the same way we do it for the URL right now.

## Context
When trying to read data from external systems like MISP, you are faced with the problem that it requires POSTing a search, which either forces you to write middleware or implement an entirely new plugin just to be able to send a POST request, which seems excessive.

## Your Environment

* Graylog Version: 3.1
* Elasticsearch Version: not applicable
* MongoDB Version: not applicable
* Operating System: not applicable
* Browser version: not applicable