Intel: Introduce Intel::seen_policy() hook

This introduces a new hook into the Intel::seen() function that allows
users to directly interact with the result of a find() call via external
scripts.

This should solve the use-case brought up by @chrisanag1985 in
discussion #3256: Recording and acting on "no intel match found".

@Canon88 was recently asking on Slack about enabling HTTP logging for a
given connection only when an Intel match occurred and found that the
Intel::match() event would only occur on the manager. The
Intel::match_remote() event might be a workaround, but possibly running a
bit too late and also it's just an internal "detail" event that might not
be stable.

Another internal use case revolved around enabling packet recording
based on Intel matches which necessarily needs to happen on the worker
where the match happened. The proposed workaround is similar to the above
using Intel::match_remote().

This hook also provides an opportunity to rate-limit heavy hitter intel
items locally on the worker nodes, or even replacing the event approach
currently used with a customized approach.

NEWS

@@ -182,6 +182,11 @@ New Functionality
   when encountering unknown/unsupported protocols. Its first use is to indicate
   ``Tunnel::max_depth`` being exceeded.
 
+- A new ``Intel::seen_policy`` hook has been introduced to allow intercepting
+  and changing ``Intel::seen` behavior:
+
+	hook Intel::seen_policy(s: Intel::Seen, found: bool)
+
 Changed Functionality
 ---------------------