Support custom fetch and WebSocket factories

[Sillyvan]

Im building a React Native app that uses capnweb. and i found something like https://github.com/margelo/react-native-nitro-fetch for a more performant fetch api that is api compatible. I could also imagine PartySocket being a useful plug and play use-case. Or even some custom wrapper.

Is this a valid idea? i could work on a PR if the idea seems good.

[Sillyvan]

I gave it a try with 5.5 commit can be seen here main...Sillyvan:capnweb:custom-fetch-websocket

would love to hear @kentonv opinion before opening a PR here.

i think fetch is very straight forward. Not so sure about the WebSocket stuff.

[kentonv]

Ehh... I'm wary of bloating the built-in transports with infinite configurability. Instead, what you can do is fork the RpcTransport implementation, make your fork work with whatever API you want, and then just use it with the rest of Cap'n Web as a custom transport.