add a unbounded endpoint that integrates unbounded into sing-box

[Jovis7]

This PR adds unbounded to sing-box as an endpoint, that can be configured to run as 3 different roles:

• client: as a sing-box outbound, that takes TCP/UDP traffic and sends it over the unbounded network (TCP/UDP over QUIC over WebRTC)
• peer: as a sing-box inbound, that receives TCP/UDP traffic (from QUIC over WebRTC) and sends it over to sing-box
• both: as both an inbound and outbound at the same time

See more details in the ticket #2035 about implementation details and decisions.

This is part of #2035 and depends on other 2 PRs to be merged first.

• quicwrapper
• unbounded

[Copilot]

Pull Request Overview

This PR integrates the Unbounded transport into sing-box as a new “unbounded” endpoint, enabling it to operate in client, peer, or dual roles over QUIC and WebRTC.

• Adds UDPOverQUICHandler to multiplex UDP traffic over QUIC datagrams, with session management and keep-alive.
• Implements Endpoint in endpoint.go to configure and run Unbounded as outbound (client), inbound (peer), or both.
• Registers the new protocol, updates options, constants, and dependencies (go.mod).

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
protocol/unbounded/udp.go	Implements UDP-over-QUIC handling, sessions, and routing
protocol/unbounded/endpoint.go	Defines the Unbounded Endpoint adapter with client/peer logic
protocol/unbounded/doc.go	Adds basic package documentation
protocol/register.go	Registers Unbounded endpoint in the registry
option/unbounded.go	Defines configuration options for the Unbounded endpoint
go.mod	Bumps Go version and adds Unbounded-related dependencies
constant/proxy.go	Adds the TypeUnbounded constant

Comments suppressed due to low confidence (2)

option/unbounded.go:6

• The WebRTCTag option is never used in Endpoint; the code generates its own UUID tags instead. If you intend to allow users to set this, wire it through createClientOptions and createPeerOptions, or remove the unused field.

	WebRTCTag  string `json:"webrtc_tag"` // might need to separate this tag for inbound (peer) and outbound (client)

protocol/unbounded/endpoint.go:205

• The Role field accepts any string at runtime but only supports client, peer, or both. Add explicit validation and return an error for unsupported values to fail fast on misconfiguration.

	switch options.Role {