feat(websocket): add headers support to websocket output

[DackJempsey]

The websocket output component does not currently support custom headers
on the WebSocket handshake, unlike the websocket input which already has
this capability.

This makes it impossible to connect to servers that require headers for
authentication or protocol negotiation (e.g. Authorization,
Sec-WebSocket-Protocol).

Proposed solution:
Add an interpolated string map headers field to the websocket output,
consistent with the existing implementation in the websocket input.
Static and Bloblang-interpolated values (e.g. ${! env("TOKEN") }) would
both be supported.

I'm opening a PR for this now.