support env vars from an existing secret

[aogier]

This simple one allows to inject environment variables from an existing secret, which is a very common pattern when using helm and let users stick to their k8s workflow. Also, keeping plaintext secrets in values file is an ultimate blocker for zulip adoption where I work.

I don't see any concern about this change, especially in respect of not changing anything in docker image inner workings but only simplifying our k8s life :)

How did you test this PR?

$ diff -u \
 <(helm template zulip . -s templates/statefulset.yaml) \
 <(helm template zulip . --set=zulip.secretEnvironment=my-secret -s templates/statefulset.yaml)
--- /proc/self/fd/11    2024-12-28 15:31:19.264000000 +0100
+++ /proc/self/fd/22    2024-12-28 15:31:19.264000000 +0100
@@ -86,6 +86,9 @@
               value: "self-signed"
             - name: ZULIP_AUTH_BACKENDS
               value: "EmailAuthBackend"
+          envFrom:
+            - secretRef:
+                name: my-secret
           resources:
             {}
           livenessProbe:

Hope this helps, ciao!