Integrate `key_value` functionality into `to_map` pipeline rule function

## What?
For clarity, it would make sense to integrate the functionality of the `key_value` function into `to_map`.
It's not very intuitive that we have a special conversion function for String to Map; and another for Collection / JSON. Particularly since `MapConversion` is described very generically as: "Converts a map-like value into a map". We don't want to break existing rules; but could we at least have `MapConversion` call `KeyValue` for String parameters? And perhaps deprecate `KeyValue`.

## Why?
This issue came up as a review comment when adding the rule builder metadata for `key_value`

## Your Environment
* Graylog Version: 5.1