Refactor API into `into_futures_async_read_seek` and `into_futures_async_read`

> Maybe there should be be two impls then. One which supports seek, and one that does not.
> We have no need for seek, for example. We just need the latest copy.
>
> Yep, that's a good idea. I think we can introduce an `into_futures_async_read_seek`. And that's will be a breaking change.
>
> Do you want to work on a fix on this?

_Originally posted by @Xuanwo in [#6995](https://github.com/apache/opendal/issues/6995#issuecomment-3646933231)_

Start with the existing API discussed in issue #6995 and trace where its async read and seek behavior is defined. Separate the interfaces into `into_futures_async_read_seek` and `into_futures_async_read`, then verify that the breaking API change is consistently reflected in the relevant callers and tests.

Written by the indexing model from the issue text.