Provide a way block-level table AMs could re-use acquire_sample_rows()

Provide a way block-level table AMs could re-use acquire_sample_rows()

While keeping API the same, this commit provides a way for block-level table
AMs to re-use existing acquire_sample_rows() by providing custom callbacks
for getting the next block and the next tuple.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20240407214001.jgpg5q3yv33ve6y3%40awork3.anarazel.de
Reviewed-by: Pavel Borisov