Enable supplementary environment variables.

[cerebrate]

Not all Mastodon configuration options are currently exposed by the Helm chart. Additionally, while it obviously isn't the job of this chart to support all the varietals of Mastodon (Glitch Edition, etc.) out there, it is convenient to be able to use it with them, and they tend to expose additional configuration options/environment variables.

This PR adds a mechanism by which additional configuration options/environment variables can be defined in a supplementary ConfigMap, and then added to the Mastodon pods using a simple addition to the values.yaml, e.g.:

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: mastodon-supplementary
data:
  ADDITIONAL_OPTION: "value"

and

mastodon:

  extraEnvironment: mastodon-supplementary

[timetinytim]

Sorry for the long delay in looking at this! This poor chart has been neglected for far too long, and I'm now finally going through all the PRs and suggestions!

[dunn]

maintenance of this repo appears to be abandoned, so unless one of us can get push permissions this probably won't go through, but all the same i would suggest using instead extraEnvVars and extraEnvFrom which are commonly used to allow providing arbitrary values directly or via an external configmap, respectively.

[timetinytim]

Apologies for the lack of activity! I've been working on infrastructure-related stuff a lot lately and recently have more time to go through these requests.

This is good call, especially since it's a common pattern. I'll take care of this right now.