Feat: Child items

Is your feature request related to a problem? Please describe.

Many structured data models require at least one grouping.
It can expressed as an abstract parent-child relation defined with two tables.

As a simple example would be the "To-Do" table for multiple projects.
A single "To-Do" table could be created/duplicated for each project
or it could be expressed in as an project-table where each entry
has a relationship to a child-table with a different schema.

Describe the solution you'd like

An option to create a single child table with a fixed relation ship.

The child records should be displayable with expandable rows
and child records should be movable between parent rows (drag&drop).

Aggregation functions (min,max,status etc.) for read only column could be provided.

Describe alternatives you've considered

The current tables app make has little difference to a spreadsheet open in collabora online.

Additional context

No response

The issue names no files, tests, or entry points. Begin by reviewing the existing tables app and defining the scope for fixed parent-child relations, expandable child rows, moving children between parents, and read-only aggregation; done requires an agreed design and working behavior for the requested capabilities.