feat: use tpl for extraEnvs for app deployment

[Gazler]

For a custom actions server, it can be desirable to leverage variables
defined in the values, for example something like:

app:
    extraEnvs:
       - name: "RASA_FULLNAME"
         value: "{{ include \"rasa-x.fullname\" . }}

To allow this, extraEnvs are now iterated over with the tpl
function, so any template strings defined in values.yml are executed.

[Gazler]

For reference, the use case I had required calling the rasa API from a custom action server, so I needed something like:

    extraEnvs:
       - name: "RASA_PRODUCTION_HOST"
         value: "{{ include \"rasa-x.fullname\" . }}-{{ .Values.rasa.versions.rasaProduction.serviceName }}.{{ .Release.Namespace }}.svc:{{ .Values.rasa.port }}"

[sara-tagger]

Thanks for submitting a pull request 🚀 @amn41 will take a look at it as soon as possible ✨