Add gpg configuration settings (#343)

### Description of the change

This PR adds support for gpg key setup. It allows to pass the gpg private key content inline inside `values.yaml` or refer to an existing secret containing the key content data.

### Benefits

Administrators don't need to manually setup the gpg environment from inside a running container. It also eliminates the breaking change of Gitea 1.17 regarding `[git].HOME` as the `GNUPGHOME` environment variable is used consistently to relocate the `.gnupg` directory to its former location.

### Applicable issues

  - fixes #107

### Additional information

This PR add the first unit tests to this Helm Chart, ensuring templating integrity for signing related configuration.

### 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)

Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-authored-by: pat-s <pat-s@noreply.gitea.io>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/343
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-committed-by: justusbunsi <justusbunsi@noreply.gitea.io>

templates/gitea/gpg-secret.yaml

@@ -0,0 +1,16 @@
+{{- if .Values.signing.enabled -}}
+{{- if and (empty .Values.signing.privateKey) (empty .Values.signing.existingSecret) -}}
+  {{- fail "Either specify `signing.privateKey` or `signing.existingKey`" -}}
+{{- end }}
+{{- if and (not (empty .Values.signing.privateKey)) (empty .Values.signing.existingSecret) -}}
+apiVersion: v1
+kind: Secret
+metadata:
+  name: {{ include "gitea.gpg-key-secret-name" . }}
+  labels:
+    {{- include "gitea.labels" . | nindent 4 }}
+type: Opaque
+data:
+  privateKey: {{ .Values.signing.privateKey | b64enc }}
+{{- end }}
+{{- end }}