Event: Deprecate default network timestamp metadata

This deprecates the Event constructor and the ``ts`` parameter of Enqueue()
Instead, versions are introduced that take a detail::MetadataVectorPtr which
can hold the network timestamp metadata and is meant to be allocated by the
caller instead of automatically during Enqueue() or within the Event
constructor.

This also introduces a BifConst ``EventMetadata::add_network_timestamp`` to
opt-in adding network timestamps to events globally. It's disabled by
default as there are not a lot of known use cases that need this.

scripts/base/init-bare.zeek

@@ -592,6 +592,15 @@ export {
 		id: EventMetadata::ID; ##< The registered :zeek:see:`EventMetadata::ID` value.
 		val: any;              ##< The value. Its type matches what was passed to :zeek:see:`EventMetadata::register`.
 	};
+
+	## Add network timestamp metadata to all events.
+	##
+	## Adding network timestamp metadata affects local and
+	## remote events. Events scheduled have a network timestamp
+	## of when the scheduled timer was supposed to expire, which
+	## might be a value before the network_time() when the event
+	## was actually dispatched.
+	const add_network_timestamp: bool = F &redef;
 }
 
 module FTP;