Add getClientRects to fabric fragment instance (#34545)

Stacked on #34544 

We only have getBoundingClientRect available from RN currently. This
should work as a substitute for this case because the equivalent of
multi-rect elements in RN is a nested Text component. We only include
the rects of top-level host components here so we can assume that
calling getBoundingClientRect on each child is the same result.

Tested in react-native with Fantom.