Switch _promoted_indexes storage in partition_index_page from
managed_vector to chunked_managed_vector to avoid large contiguous
allocations.
Avoid allocation failure (or crashes with --abort-on-internal-error)
when large partitions have enough promoted index entries to trigger a
large allocation with managed_vector.
Fixes: SCYLLADB-1315
Closesscylladb/scylladb#29283
(cherry picked from commit 2d2ff4fbda)
Closesscylladb/scylladb#29304