perf(rt): add ByteBuffer-backed stream to avoid Data copies on the NI…

[jbelkins]

…O path

The SwiftNIO transport round-trips every streamed byte through Foundation.Data, which forces extra copies that NIO-native SDKs avoid. On download the response bridge copies ByteBuffer -> [UInt8] -> Data -> BufferedStream (3 copies/chunk); on upload it copies Data -> a freshly allocated ByteBuffer (1 alloc + 1 copy/chunk). The write side is also synchronous and unbounded, so a fast producer cannot be throttled and the in-memory buffer can grow without limit.

This change is additive only; no existing public API changes:

• Smithy: add WriteableStream.writeAsync(contentsOf:) as a protocol-extension default that bridges to the existing synchronous write, so every current conformer compiles unchanged. (A distinct name is used rather than an async overload of write(contentsOf:), which would be source-breaking because an async context would prefer the overload and force existing call sites to await.)
• SmithySwiftNIO: add ByteBufferStream, a Stream-conforming type backed by a FIFO of NIOCore.ByteBuffers. Reads vend ByteBuffer slices via readSlice (advancing the readerIndex, no memmove); writes keep the producer's ByteBuffer (copy-on-write). The async write applies high/low-watermark backpressure so the buffer stays bounded. The Data-returning protocol methods still work, performing a single boundary copy only for legacy consumers.
• SmithySwiftNIO: SwiftNIOHTTPClientStreamBridge uses ByteBufferStream on the response path and prefers a zero-copy ByteBuffer slice on the request path via an as? downcast, leaving the default Data path unchanged.

Measured with an identical-work consumer (release build): the download path goes from ~1230 MiB/s to ~6640 MiB/s (5.4x), within ~5% of the zero-copy ceiling; upload improves ~1.4x.

Known follow-ups (not in this change): task-cancellation handlers on the async suspensions, and migrating the checksum/chunked middlewares that currently hard-code BufferedStream.

Issue #

Description of changes

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

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