[ws] [ECR] Ability to differentiate response based on triggering event

## Use case:

I have an element that can either be swiped left or swiped right. It’s `hx-trigger` looks like this:

```
hx-trigger='swiped-left, swiped-right'
```

On the server, I can’t differentiate whether it was a swipe left or swipe right.

## Suggested enhancement

Add the `triggeringEvent`’s `type` to the `HEADERS` that are sent with the message so I can differentiate between a swipe left and swipe right on my element.

e.g., the header could be:

```
'HX-Trigger-Event': 'swiped-right'
```

Start at the websocket extension entry point that handles hx-trigger events and assembles HEADERS for messages. Trace how triggeringEvent is available there and verify the header format against the existing message flow. Done means the triggering event type is sent as HX-Trigger-Event so the server can distinguish swiped-left from swiped-right.

Written by the indexing model from the issue text.