Support serializing/deserializing Response objects with a websocket

https://github.com/cloudflare/capnweb/blob/v0.8.0/src/serialize.ts#L303-L306

This issue is just a quick note to say that websockets support would be REALLY valuable to us

We make heavy use of capnweb in our e2e tests - where our node-based vitest test runner provides a small web server that is then exposed to some public service under test via a captun.sh tunnel (which is just a v thin layer around capnweb)

But we can't use this mechanism when we want to spin up a service that supports websockets - which we want to do quite often. In fact, the thing we'd really want to do quite often is this

- We have a publicly deployed service we'd like to test
- This service makes capnweb connections to "stream processors" via public URLs using capnweb's websockets transport
- We want to quickly spin up lots of stream processors in our vitest tests and allow our publicly deployed service under test to talk to them

So we'd basically be doing capnweb via websockets transport tunneled through capnweb websockets transport. Fun!