Make body stream writer sendable

[glbrntt]

Motivation:

The body stream writer can be sent across isolation domains so should be sendable.

Modifications:

• Make it explicitly sendable
• Add appropriate preconcurrency annotations
• Wrap an iterator from swift-algorithms as it hasn't yet annotated its types with Sendable

Result:

Body stream writer is sendable

[glbrntt]

This is expected:

1 breaking change detected in AsyncHTTPClient: 💔 API breakage: constructor HTTPClient.Body.StreamWriter.init(closure:) is now with @preconcurrency