signatures: Support custom event via [event_name] syntax

This change allows to specify a per signature specific event, overriding
the default signature_match event. It further removes the message
parameter from such events if not provided in the signature.

This also tracks the message as StringValPtr directly to avoid
allocating the same StringVal for every DoAction() call.

Closes #3403

testing/btest/signatures/custom-event-errors.zeek

@@ -0,0 +1,33 @@
+# @TEST-DOC: Using the wrong paramters for custom signature events.
+#
+# @TEST-EXEC-FAIL: zeek -b -s id -r $TRACES/chksums/ip4-udp-good-chksum.pcap %INPUT >id.out
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+
+@TEST-START-FILE id.sig
+signature udp-proto {
+  ip-proto == 17
+  event [wrong_signature2] "id"
+}
+
+signature udp-proto2 {
+  ip-proto == 17
+  event [wrong_signature3]
+}
+
+signature udp-proto3 {
+  ip-proto == 17
+  event [wrong_signature4] "not a count"
+}
+
+signature udp-proto4 {
+  ip-proto == 17
+  event [non_existing_event]
+}
+
+@TEST-END-FILE
+
+event wrong_signature2(state: signature_state, data: string) { }
+
+event wrong_signature3(state: signature_state, msg: string, data: string) { }
+
+event wrong_signature4(state: signature_state, msg: count, data: string) { }