Add support for ServiceAccount configuration (#451)

### Description of the change

This adds a new values object `serviceAccount`, that allows creating a dedicated ServiceAccount with the Helm Release into the cluster. It supports all common options like labels, annotations, name override (or referring to an externally created ServiceAccount), auto-mount token, image pull secrets.

It supersedes the stale PR #357.

### Benefits

Users can deploy Gitea with more fine-tuned security settings.

### Applicable issues

- related to #448

### Additional information

I've bumped the helm-unittest plugin in the CI build, to be able to use the `exists` and `notExists` feature in the new tests.

### 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)

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/451
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-committed-by: justusbunsi <sk.bunsenbrenner@gmail.com>

templates/_helpers.tpl

@@ -311,3 +311,7 @@ https
 {{- define "gitea.gpg-key-secret-name" -}}
 {{ default (printf "%s-gpg-key" (include "gitea.fullname" .)) .Values.signing.existingSecret }}
 {{- end -}}
+
+{{- define "gitea.serviceAccountName" -}}
+{{ .Values.serviceAccount.name | default (include "gitea.fullname" .) }}
+{{- end -}}