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/init/init_directory_structure.sh.yaml

@@ -0,0 +1,53 @@
+suite: Init template
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/init.yaml
+tests:
+  - it: skips gpg script block for disabled signing
+    asserts:
+      - equal:
+          path: stringData.[init_directory_structure.sh]
+          value: |-
+            #!/usr/bin/env bash
+
+            set -euo pipefail
+
+            set -x
+            chown 1000:1000 /data
+            mkdir -p /data/git/.ssh
+            chmod -R 700 /data/git/.ssh
+            [ ! -d /data/gitea/conf ] && mkdir -p /data/gitea/conf
+
+            # prepare temp directory structure
+            mkdir -p "${GITEA_TEMP}"
+            chown 1000:1000 "${GITEA_TEMP}"
+            chmod ug+rwx "${GITEA_TEMP}"
+  - it: adds gpg script block for enabled signing
+    set:
+      signing.enabled: true
+    asserts:
+      - equal:
+          path: stringData.[init_directory_structure.sh]
+          value: |-
+            #!/usr/bin/env bash
+
+            set -euo pipefail
+
+            set -x
+            chown 1000:1000 /data
+            mkdir -p /data/git/.ssh
+            chmod -R 700 /data/git/.ssh
+            [ ! -d /data/gitea/conf ] && mkdir -p /data/gitea/conf
+
+            # prepare temp directory structure
+            mkdir -p "${GITEA_TEMP}"
+            chown 1000:1000 "${GITEA_TEMP}"
+            chmod ug+rwx "${GITEA_TEMP}"
+
+            if [ ! -d "${GNUPGHOME}" ]; then
+              mkdir -p "${GNUPGHOME}"
+              chmod 700 "${GNUPGHOME}"
+              chown 1000:1000 "${GNUPGHOME}"
+            fi