Support thousands of rows in RepeatingGroup

[olemartinorg]

Source:

• https://altinn.slack.com/archives/C02EJ9HKQA3/p1727189717723409

After the hierarchy rewrite, although the work there meant to improve performance, loading thousands of rows is still not performing optimally. I would assume that fixing this properly requires hard architecture decisions in frontend. Making an issue for this here to have something in the backlog, but the broad architecture strokes are absolutely still a subject we're discussing.