support yaml for providers.file.content

### Welcome! - [x] Yes, I've searched similar issues on [GitHub](https://github.com/traefik/traefik/issues) and didn't find any. - [x] Yes, I've searched similar issues on the [Traefik community forum](https://community.traefik.io) and didn't find any. ### What do you want to achieve with this Chart ? It would be easier for me to lint/validate my file-provider config if it could be actual yaml instead of yaml-in-yaml-string (syntax highlighting and validation, etc.), i.e. instead of: ```yaml file: enabled: true content: | http: middlewares:... ``` ```yaml file: enabled: true content: http: middlewares:... ``` Adding it to the configmap should be only a change from: ``` {{ content | toYaml | nindent 4 }} ``` to: ``` {{ content | toYaml | quote | nindent 4 }} ``` In order to be backward-compatible, we could do this as a new `contentYaml` or `contentData` field. Either that or a switch on the type of `content`, whichever is preferable. Would a PR be welcome? And if so, any preference for adding a switch for the type of `content` or a sibling field?