RFC: Allow topic extraction via json::decode()

[awelzel]

Hey @Neverlord - I'm attempting to use broker::format::json::v1::decode() to decode Zeek events sent by external clients, but implemented outside of the Broker layer. The Broker WebSocket format defines the "topic" a client publishes to, to be part of the top-level JSON object.

This PR changes decode() to optionally support extracting the topic, too. Does this feel too ad-hoc? I realize decode() can decode non-events, too, so maybe adding a separate helper would be the better approach?

It's this part from the internal/json_client.cc, but it has a parsed JSON representation available:

broker/libbroker/broker/internal/json_client.cc

Lines 140 to 142 in 5847b2a

	auto maybe_msg = data_envelope::deserialize(
	id, endpoint_id::nil(), defaults::ttl,
	caf::to_string(obj.value("topic").to_string()), buf.data(), buf.size());