Add 'additionalContainers' value for flyteadmin

[andrewwdye]

This change adds an optional value to append containers to the flyteadmin containters list. This can be used to co-deploy sidecar functionality.

Verified that the following local changes

   # -- Appends additional containers to the containers list. May include template values.
-  additionalContainers: {}
+  additionalContainers:
+  - command:
+    - flyteadmin
+    - --config
+    - "{{ .Values.flyteadmin.configPath }}"
+    - ...

Produce the expected output

--- a/deployment/eks/flyte_aws_scheduler_helm_generated.yaml
+++ b/deployment/eks/flyte_aws_scheduler_helm_generated.yaml
@@ -965,6 +965,11 @@ spec:
           name: clusters-config-volume
         - mountPath: /etc/secrets/
           name: admin-secrets
+      - command:
+        - flyteadmin
+        - --config
+        - '/etc/flyte/config/*.yaml'
+        - '...'
       serviceAccountName: flyteadmin
       volumes:
         - name: db-pass