io: add a CopyBuffer::with_size method

[aym-v]

This PR is a proposal to close #3722.
It adds a new CopyBuffer::with_size method which allows users to set the size of the buffer if the default size of 2048 is inadequate.

[taiki-e]

This PR is a proposal to close #3722.
It adds a new CopyBuffer::with_size method which allows users to set the size of the buffer if the default size of 2048 is inadequate.

What this PR adds is a method on a private type, so users will not be able to specify buffer size yet.
To implement the functionality proposed by #3722, I think you have to add a new public function like #3572.

[aym-v]

@taiki-e Thank you for your help, a public method is now exposed with the latest commit

[rneswold]

With each new release of tokio, I keep hoping to find this feature added. Is there a plan to add this soon?

[taiki-e]

@rneswold As said in #3722, the current implementation of this PR is equivalent to copy_buf + BufReader.

let mut reader = BufReader::with_capacity(size, reader);
copy_buf(&mut reader, &mut writer).await?;

Given that equivalent code is already available on the released versions, this feature request is not necessarily a high priority, and I think it would be better to have a discussion about alternative implementation before merging this.