Invocation events via websocket

[vigoo]

Currently we are only emitting the following worker events through websocket:

• stdout
• stderr
• log

With the new --connect feature of golem-cli worker invoke, it's confusing that there is no way to know which log messages were emitted by a particular invoation. To solve this, we could extend the set of worker events and emit a "invocation start" and "invocation end" event through the broadcast channel (ending up in the websocket connection of connect). If these messages contain the idempotency key of the invocation, then clients like golem-cli can filter out messages before/after the invocation (and can await the last emitted message instead of cancelling the stream as soon as we get a result value).

[vigoo]

This is already implemented.