Provide a way to create a "combined" parameter

Problem/Motivation

For some use cases it may be useful to create a pre-defined set of a combination of parameter plugins. Meaning creating a new parameter type via configuration, that itself then contains a combination of other parameter types.

Example: In order to manage templates of E-Mail templates that always consist of a subject and body, it would be more convenient to manage it as one single parameter, instead of having one parameter for the subject and another one for the body.

Steps to reproduce

Proposed resolution

Needs to be evaluated what could be a possible solution path. Somehow the user needs to be able to define the properties with a machine name, and then the corresponding parameter plugin to use for that property.

Remaining tasks

User interface changes

API changes

Data model changes