feat(s2n-quic-core): add ReceiveBuffer::copy_into_buf

[camshaft]

Description of changes:

For an application that needs to copy data out into, for example, a tokio::io::ReadBuf, the current ReceiveBuffer::pop interface is less than ideal. The way it is implemented, it assumes that the BytesMut will be passed around for copy-avoidance. Splitting bytes off from other chunks only to copy it and drop the new reference requires shared promotion and refcount increment/decrement.

This change, instead, provides a copy_into_buf function that avoids the split calls and instead uses BufMut::advance_mut, which avoids the previously-mentioned downsides.

Call-outs:

I updated the benchmarks to use the copy_into_buf function instead of pop to avoid oversampling on the pop, especially since the benchmarks just discard the received chunk.

Testing:

I've included a unit test showing it all working. Most of the implementation is shared with pop via the internal pop_transform method.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.