support executing configurable admin commands via SIGUSR2

Signed-off-by: Jason Volk <jason@zemos.net>

conduwuit-example.toml

@@ -1362,6 +1362,13 @@
 #
 #admin_execute_errors_ignore = false
 
+# List of admin commands to execute on SIGUSR2.
+#
+# Similar to admin_execute, but these commands are executed when the
+# server receives SIGUSR2 on supporting platforms.
+#
+#admin_signal_execute = []
+
 # Controls the max log level for admin command log captures (logs
 # generated from running admin commands). Defaults to "info" on release
 # builds, else "debug" on debug builds.