Support for dollar sign in message field names

## What?
Field names which contain a dollar sign (like `field_name$`) are currently not supported, because we are using GELF internally which does not support field names with this character.

Here is the regex for valid keys:
https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/plugin/Message.java#L193

This can be confusing in some cases because these fields show up e.g. when testing a JSON extractor for an input, but they are not part of the message after the injection.

At some point in the future we could enforce the GELF spec only for the GELF codec.

## Your Environment

* Graylog Version: 4.0