Add DTLS (Datagram Transport Layer Security) Support

[fpseverino]

NIOSSL currently provides TLS support for TCP connections, but does not support DTLS (Datagram Transport Layer Security), which is required for secure communication over UDP.

Adding DTLS support would significantly broaden the applicability of swift-nio-ssl, enabling use cases like:

• WebRTC components
• Real-time media / VoIP
• IoT and constrained devices
• Custom UDP-based secure protocols

BoringSSL already provides DTLS primitives (as far as I can tell for all versions)

[Lukasa]

Yep, this is a reasonable enhancement.