Signals MVP

[lucksus]

PR summary

This adds an HDK function:

hdk::emit_signal("my signal", args.into());

which will send Signal::UserSignals over the signal channel and websocket interface to the UI where it can be listened to like so:

const { onSignal } = await this.webClientConnect({url})

onSignal((msg) => {
  console.log(msg.signal) // -> { signal_type: 'User', name: 'test-signal', arguments: '{"message":"test message"}' }
})

So it is a bare-bones signal implementation without all the checks and DNA defintions that we were hinting towards in the Signals ADR.

But it should be enough to get Basic-Chat rolling with a UI that refreshes automatically and without polling :) (pinging @philipbeadle and @willemolding)

Is using some changes to Diorama to actually test if signals arrive at the interface client.

followups

• Use this in Basic-Chat or Hylo :)
• Implement the rest of Signals as described in the ADR which will make it possible to introspect about signals and set them up as bridges etc.

changelog

• this is a code change that effects some consumer (e.g. zome developers) of holochain core so it is added to the CHANGELOG-UNRELEASED.md (linked above), with the format - summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

[willemolding]

This is amazing. So excited to play around with it.

[zippy]

This looks great. Approving for functionality, but I notice that there's nothing in the MD Book. Can you please at least add a place-holder so that we don't forget?

[philipbeadle]

Works great in basic chat