DRY improvements (#664)

### Description of the change

Adding support for DRY principle (via use of the TPL function) to the PVC storage class and the ingress class

### Benefits

It allows to reference a variable into another one to avoid duplicating them (or using YAML anchors).
It is useful and valuable when including Gitea into an umbrella chart with multiple components and to have a single variable while the components variable only refer to that single one.

Example 1
```
global:
  persistence:
    storageClass: "storage-class"
persistence:
  storageClass: "{{ .Values.global.persistence.storageClass }}"
```
This results in having `spec.storageClassName` equal to `storage-class` in the PVC object

Example 2
```
global:
  ingress:
    className: "ingress-class"
ingress:
  className: "{{ .Values.global.ingress.className}}"
```
This results in having `spec.ingressClassName` equal to `ingress-class` in the Ingress object

### Possible drawbacks

N/A

### Checklist

- [X] Templating unittests are added

Co-authored-by: 212597596 <cedric.henry@ge.com>
Co-authored-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/664
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: Ceddaerrix <ceddaerrix@noreply.gitea.com>
Co-committed-by: Ceddaerrix <ceddaerrix@noreply.gitea.com>

unittests/pvc/pvc-configuration.yaml

@@ -0,0 +1,19 @@
+suite: PVC template
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/pvc.yaml
+tests:
+  - it: Storage Class using TPL
+    set:
+      global.persistence.storageClass: "storage-class"
+      persistence.enabled: true
+      persistence.create: true
+      persistence.storageClass: "{{ .Values.global.persistence.storageClass }}"
+    asserts:
+      - isKind:
+          of: PersistentVolumeClaim
+      - equal:
+          path: spec.storageClassName
+          value: "storage-class"