swift-nio-http2: Missing CR/LF/NUL validation in header values

Summary SwiftNIO HTTP/2 was missing validation on inbound HEADERS frames that let CR, LF, NUL, SP and other control characters reach an HTTP/1.1 backend through NIOHTTP2's HTTP/2-to-HTTP/1 codec, enabling HTTP request smuggling or response splitting. Impact Two related gaps in inbound header...