Allowing Custom Labels in SVC Templates (#597)

### Description of the change
The change allows users of this chart to specify custom labels for the HTTP & SSH Services that get deployed. They are optional and if given are added to the standard list of labels.

### Benefits
Certain use cases require labelling services to be able to select them in other places. Specifiying them directly in the helm chart avoids having to label by hand everytime.

Concrete Use Case Example: Cilium Layer 2 Announcements require selecting services to announce via labels (see [docs](https://docs.cilium.io/en/stable/network/l2-announcements/#service-selector)). I would like to add a label to the SSH service, but not the HTTP Service (which is handled via an ingress).

### Possible drawbacks
I don't see any, using this feature is optional :)

### Checklist

- [X] Parameters are documented in the `values.yaml` and added to the `README.md` using [readme-generator-for-helm](https://github.com/bitnami-labs/readme-generator-for-helm)
- [X] Breaking changes are documented in the `README.md`
- [X] Templating unittests are added

Co-authored-by: Remo Geissbühler <git@remogeissbuehler.ch>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/597
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: remogeissbuehler <remogeissbuehler@noreply.gitea.com>
Co-committed-by: remogeissbuehler <remogeissbuehler@noreply.gitea.com>

unittests/deployment/svc-configuration.yaml

@@ -0,0 +1,51 @@
+suite: ssh-svc / http-svc template (Services configuration)
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/ssh-svc.yaml
+  - templates/gitea/http-svc.yaml
+tests:
+  - it: supports adding custom labels to ssh-svc
+    template: templates/gitea/ssh-svc.yaml
+    set:
+      service:
+        ssh:
+          labels:
+            gitea/testkey: testvalue
+    asserts:
+      - equal:
+          path: metadata.labels["gitea/testkey"]
+          value: "testvalue"
+
+  - it: keeps existing labels (ssh)
+    template: templates/gitea/ssh-svc.yaml
+    set:
+      service:
+        ssh:
+          labels: {}
+    asserts:
+      - exists:
+          path: metadata.labels["app"]
+
+  - it: supports adding custom labels to http-svc
+    template: templates/gitea/http-svc.yaml
+    set:
+      service:
+        http:
+          labels:
+            gitea/testkey: testvalue
+    asserts:
+      - equal:
+          path: metadata.labels["gitea/testkey"]
+          value: "testvalue"
+
+  - it: keeps existing labels (http)
+    template: templates/gitea/http-svc.yaml
+    set:
+      service:
+        http:
+          labels: {}
+    asserts:
+      - exists:
+          path: metadata.labels["app"]