support merge io read api by settings

[sundy-li]

opendal could support batch read API that coalesces the ranges into merge ranges that send merged requests into storage.

Something like

1. https://docs.rs/object_store/latest/src/object_store/lib.rs.html#581-588
2. https://github.com/datafuselabs/databend/blob/a98335d33e7abfd34189e7f32c06ab34d53c64d0/src/query/storages/fuse/src/io/read/block/block_reader_merge_io_async.rs

Notice that the seek of some backends (eg: fs) is not costable, so it's better to read synchronized than join futures.

[Xuanwo]

Implemented by #4488