Utility to create proxy hooks

[pi0]

Describe the feature

Context: nitrojs/nitro#2886

A new createWebSocketProxy({ target: 'ws://external-url' }) would allow users (and frameworks) to create runtime-agnostic hooks that proxy connections to a remote backend.

Implementation details depend on CrossWS hooks and we might benefit from some native runtime-aware possibilities also (mainly to speed up or allow platform-specific protocols for example) therefore CrossWS core itself is probably the best place to develop and maintain this utility.