EventRegistry/zeek.bif/init-bare: Add event metadata infrastructure

Introduce a new EventMetadata module and members on EventMgr to register
event metadata types.

testing/btest/core/event-metadata/register-type-errors.zeek

@@ -0,0 +1,30 @@
+# @TEST-DOC: Verify rejection of certain metadata types.
+#
+# @TEST-EXEC: unset ZEEK_ALLOW_INIT_ERRORS; zeek -b %INPUT
+# @TEST-EXEC: btest-diff .stdout
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+
+module App;
+
+export {
+	redef enum EventMetadata::ID += {
+		MY_METADATA = 1000,
+		MY_TABLE = 1002,
+		MY_VECTOR = 1003,
+	};
+}
+
+type R: record {
+	f: file;
+	a: any;
+	l: function(x: count): bool;
+};
+
+event zeek_init()
+	{
+	assert ! EventMetadata::register(MY_METADATA, any);
+	assert ! EventMetadata::register(MY_METADATA, table[count] of any);
+	assert ! EventMetadata::register(MY_METADATA, table[count] of function(x: count): bool);
+	assert ! EventMetadata::register(MY_METADATA, R);
+	assert ! EventMetadata::register(MY_METADATA, vector of R);
+	}