Repeating groups: Copy row as new row

### Description

In #107, a feature called 'repeating tables' is described. Functionality described there has been adopted as functionality in our repeating groups instead (which can also be displayed as a table), but one nifty feature has not been described elsewhere yet - copying the information in one row as the basis for a new row. Extracting this out from #107 before closing it.

We should support an opt-in feature where rows can be copied by the user into new rows. We should make sure to allow for configuration here, so that the app developer can select which parts of the data model should be copied (i.e. never copying shadow fields or prefill data).

See a demonstration where this functionality is just barely shown, in the now-outdated sketches for repeating tables:

https://user-images.githubusercontent.com/1464915/156745656-21ef6fed-2d37-420c-b2bd-d3a67e0e8c03.mov

### Additional Information

- #107

Start by reviewing the existing repeating groups implementation and the related requirements in issue #107. Define the opt-in row-copy behavior and its configuration, including which data-model fields are excluded; done means users can create a new row from an existing row without copying shadow fields or prefill data.

Written by the indexing model from the issue text.