add support for plugins

[monrax]

Add support for plugins through direct download at init time

[williamtrelawny]

@monrax Can you resolve this conflict please? I'm not familiar enough with git syntax to know how to fix it 😓

[monrax]

I just updated this branch, and rebased on top of main. All conflicts should be resolved @williamtrelawny

This PR introduces support for Graylog plugins, as well as support for custom environment variables in both graylog and datanode.

Plugins

• Given an existing PVC, by specifying the existingClaim name for a given plugin, it will be mounted to the copy-data init container at /mnt/shared/plugins/<plugin-name>. The init container will then copy all .jar files from /mnt/shared/plugins/* to the main application plugin directory (mounted at /mnt/plugin in the init container, and /usr/share/graylog/plugin in the main graylog container).
• Given a custom container image that holds any number of JAR files in its working directory, by specifying the image field with the full image reference (image: "<repo/image:tag>") for a given plugin, it will be used to start a new init container with the following command: cp *.jar /mnt/plugins/, copying all .jar files to the main application plugin directory (mounted at /mnt/plugin in the init container, and /usr/share/graylog/plugin in the main graylog container).
• Given the URL of a JAR file available in a remote location, and a SHA256 hash of that JAR file, by specifying both the url and checksum fields for a given plugin, the copy-data init container will fetch the JAR resource and check if the hashes match, and if so it will copy the file to the main application plugin directory (mounted at /mnt/plugin in the init container, and /usr/share/graylog/plugin in the main graylog container).

Env vars

For both graylog.custom and datanode.custom, custom environment variables can be specified:

• As simple name-value pairs using env.
  e.g. --set graylog.custom.env.FOO=BAR or --set datanode.custom.env.MY_FLAG=true
• As a full Kubernetes EnvVar list spec using extraEnv.

# e.g. 
graylog:
  custom:
    extraEnv:
    - name: FOO
      value: BAZ
    - name: MADE_UP_PASSWORD
      valueFrom:
        secretKeyRef:
          name: mysecret
          key: password

In the case of a given key specified using both env and extraEnv, the latter takes preference.

---

All of this has been tested with both microk8s and EKS.

[alix-graylog]

Looks good to me. I don't have a great way to test this on my end, but I this looks solid.