Offer ResponseSender.Writer RawSpan APIs

We should offer RawSpan APIs for the ResponseSender.Writer to avoid an extra copy when it takes the container and copies it to a ByteBuffer. If we have a RawSpan API we can bypass that allocation for every write. I verified this in Vapor 5 where currently we have to stage every chunk into a UniqueArray. I've tested this and it reduces the copy for every write, which can add up when streaming large responses. It reduces it down to a single copy in the pipeline. Will submit a PR with the new APIs.

We can write overloads to do a borrow instead of the copy:

public mutating func write(_ bytes: RawSpan) async throws(WriteFailure)
public consuming func finish(bytes: RawSpan, finalElement: consuming HTTPFields?) async throws(WriteFailure)

Note that we'd need an overload for finish as well which covers non-streaming cases.

Start at ResponseSender.Writer and trace how writes currently stage chunks in a UniqueArray before copying to a ByteBuffer. Compare the existing write and finish APIs, including the non-streaming finish path. Done means RawSpan overloads cover writing and finishing while reducing the pipeline to a single copy.