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>

unittests/gpg-secret/signing-disabled.yaml

@@ -0,0 +1,13 @@
+suite: GPG secret template (signing disabled)
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/gpg-secret.yaml
+tests:
+  - it: renders nothing
+    set:
+      signing.enabled: false
+    asserts:
+      - hasDocuments:
+          count: 0