refactor: Relax bounds on Writer::{sink, copy}

[huonw]

This adjusts the trait bounds on the input stream (for sink) or reader (for copy) for the opendal::Writer type to be less strict, and thus allow more types to be passed in.

In particular, the current implementations of these functions do not use any of the additional "information"/requirements that Send + Sync + Unpin + 'static imply. The 'static bound is particularly restrictive, as it stops one being able to pass in any borrowed data, such as a &mut dyn AsyncRead object or similar. Without these bounds, one can pass in more readers or more streams, more simply.

There is a downside to this change: if these implementations wanted to change in future to use any of these (e.g. a threaded implementation of copy or something), that becomes a breaking change. I would suggest that's okay: any fancy implementation like that would be best offered in addition to these simple ones, exactly because they'll need to impose additional restrictions on their input types.

(Thank you for OpenDAL!)

[Xuanwo]

Thanks a lot!