### 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>
