[PaaS] Improve OpenUI extensions to be able to save multiple fields/values

Taking for example the Widget Picker for the Pathways and Recommendations component, it is currently not possible to infer the algorithm of the selected widget because fields cannot read the information from each other. This creates a dis-jointed user experience, since it is required for the user to know the algorithm of the widget selected and then select it from a dropdown list.
It would be nice to have the possibility to read values from multiple fields in order to fulfil this use-case.

As a more specific example, when a user is selecting widget algorithm and widget ID through an OpenUI Extension plugin, the plugin wants to store two different field values - one for widget algorithm and the other for widget ID.
There could be a workaround by storing a JSON-like string in a single field value for both values.
But it might not be so ideal in the future - for example, when maintaining existing page/component param values, querying data by a json string is not so intuitive as querying by separate explicit values.

By design, the OpenUI field extension is only for single value a field, so this use case might be so fit to the current design.

But it would be really good if OpenUI extension supports this kind of use case anyway: An (new) OpenUI field extension plugin to be able to store multiple field values in separate document/compound fields.