Endpoint for backing up and showing full graylog configuration

## **Basic idea:**
I'd be really nice to have an easy to use endpoint which with we could backup the whole stack's configuration (from mongoldb, Elasticsearch's and the graylog config file). Also I'd be nice to have an endpoint where we could dump the whole graylog configuration as Json or YAML

## **How It'd work:**
Three new endpoints. One for backing up the graylog configuration with an option to include the elasticSearch config + stored logs.
The second would be used to restore these backups.
The last one could be an endpoint which dumps the current full configuration of graylog + elasticSearch, so it would be a piece of cake to test it against infrastructure testers (for example: terratest)

## **Why?**
It is cumbersome to backup graylog, and as it is a vital part of any application, this should be easier and less error prone plus should be easy to automate.
The config dump endpoint would help testability greatly.

## **Your Environment**

* Graylog Version: 4.0.0
* Elasticsearch Version: 7.10.2
* MongoDB Version: 4.2
* Operating System: Red Hat and MacOS
* Browser version: Safari Version 14.0.3 (16610.4.3.1.7)

## **Further question(s)**
Is it possible that I'm opening a feature branch and try to implement it? As I read if I want to contribute I need to talk to the core team, I'm guessing it's done by creating this ticket. (And I was advised in the freeNode chat) :)