* TxtDataHelpers.h, with standard TXT sub-types

* Terminal chat: added "time <epoch-secs>"
* repeater: now recognises either TXT_TYPE_PLAIN or TXT_TYPE_CLI_DATA for CLI commands
* Room server: now uses TXT_TYPE_SIGNED_PLAIN (for push/outbound), recognises TXT_TYPE_PLAIN (to add a Post), and TXT_TYPE_CLI_DATA for CLI command

src/helpers/TxtDataHelpers.h

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+#define TXT_TYPE_PLAIN          0    // a plain text message
+#define TXT_TYPE_CLI_DATA       1    // a CLI command
+#define TXT_TYPE_SIGNED_PLAIN   2    // plain text, signed by sender