feat(immich): support read-only external libraries

Add immich_external_libraries variable to mount host paths into the
server container, intended for use with Immich's External Libraries
feature. Mounts are read-only; the in-container mount_path must be
used when registering the library in the Immich UI.

roles/immich/templates/immich.yaml.j2

@@ -45,6 +45,11 @@ spec:
           readOnly: true
         - name: immich-data
           mountPath: /data
+{% for lib in immich_external_libraries %}
+        - name: ext-{{ lib.name }}
+          mountPath: {{ lib.mount_path }}
+          readOnly: true
+{% endfor %}
       livenessProbe:
         httpGet:
           path: /api/server/ping
@@ -83,6 +88,12 @@ spec:
       hostPath:
         path: {{ immich_upload_location }}
         type: Directory
+{% for lib in immich_external_libraries %}
+    - name: ext-{{ lib.name }}
+      hostPath:
+        path: {{ lib.host_path }}
+        type: Directory
+{% endfor %}
     - name: model-cache
       persistentVolumeClaim:
         claimName: immich-model-cache