feat(deployment): support further initContainers

The following patch intoduce the dictionaries pre and postExtraInitContainers.
The dictionaries can be used to specify further initContainers before and after
the gitea initializing process. For example:

```yaml
postExtraInitContainers:
- name: foo
  image: docker.io/library/busybox:latest
preExtraInitContainers:
- name: bar
  image: docker.io/library/busybox:latest
```

unittests/helm/deployment/extraInitContainers.yaml

@@ -0,0 +1,59 @@
+suite: deployment template
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+- templates/gitea/deployment.yaml
+- templates/gitea/config.yaml
+tests:
+- it: Render the deployment (default)
+  asserts:
+  - hasDocuments:
+      count: 1
+    template: templates/gitea/deployment.yaml
+  - lengthEqual:
+      path: spec.template.spec.initContainers
+      count: 3
+    template: templates/gitea/deployment.yaml
+
+- it: Render the deployment (signing)
+  set:
+    signing.enabled: true
+  asserts:
+  - hasDocuments:
+      count: 1
+    template: templates/gitea/deployment.yaml
+  - lengthEqual:
+      path: spec.template.spec.initContainers
+      count: 4
+    template: templates/gitea/deployment.yaml
+
+- it: Render the deployment (extraInitContainers)
+  set:
+    postExtraInitContainers:
+    - name: foo
+      image: docker.io/library/busybox:latest
+    preExtraInitContainers:
+    - name: bar
+      image: docker.io/library/busybox:latest
+    signing.enabled: true
+  asserts:
+  - hasDocuments:
+      count: 1
+    template: templates/gitea/deployment.yaml
+  - lengthEqual:
+      path: spec.template.spec.initContainers
+      count: 6
+    template: templates/gitea/deployment.yaml
+  - contains:
+      path: spec.template.spec.initContainers
+      content:
+        name: foo
+        image: docker.io/library/busybox:latest
+    template: templates/gitea/deployment.yaml
+  - contains:
+      path: spec.template.spec.initContainers
+      content:
+        name: bar
+        image: docker.io/library/busybox:latest
+    template: templates/gitea/deployment.yaml