add new method for io::copy that take buffer size

[cssivision]

as discussing in #3572, add a new issue to trace it.

currently io::copy take 2k bytes buffer, there isn't any way of making the buffer bigger now. maybe we can add a new method to take byffer size as input.

[dyxushuai]

In addition, maybe the buffer can be inited by MaybeUninit for some performance issues

[taiki-e]

How is this different from copy_buf + BufReader?
When I look at #3726's implementation, I feel it does the equivalent thing.

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

[Darksonn]

Huh, that's an interesting point. It could even make sense to entirely replace the implementation to use that. I'll have to think about this.

[Darksonn]

As far as I can tell, using BufReader like this would leave #3694 open, and it doesn't seem like we can fix that issue while also using that implementation unless we add a new method to AsyncBufRead.

[Hanaasagi]

When I use copy_bidirectional function, I find that the buf_size is 8k and cannot be adjusted. 👀

tokio/tokio/src/io/util/copy.rs

Lines 18 to 28 in 3abe877

	impl CopyBuffer {
	pub(super) fn new() -> Self {
	Self {
	read_done: false,
	need_flush: false,
	pos: 0,
	cap: 0,
	amt: 0,
	buf: vec![0; super::DEFAULT_BUF_SIZE].into_boxed_slice(),
	}
	}