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>

values.yaml

@@ -253,9 +253,17 @@ initPreScript: ""
 #
 ## @param signing.enabled Enable commit/action signing
 ## @param signing.gpgHome GPG home directory
+## @param signing.privateKey Inline private gpg key for signed Gitea actions
+## @param signing.existingSecret Use an existing secret to store the value of `signing.privateKey`
 signing:
   enabled: false
   gpgHome: /data/git/.gnupg
+  privateKey: ""
+  # privateKey: |-
+  #   -----BEGIN PGP PRIVATE KEY BLOCK-----
+  #   ...
+  #   -----END PGP PRIVATE KEY BLOCK-----
+  existingSecret: ""
 
 ## @section Gitea
 #