Terminal: change command-line parser (#2247)

Change the underlying command line handling:
- switch to a custom parser, inspired by redis / sds
- update terminalRegisterCommand signature, pass only bare minimum
- clean-up `help` & `commands`. update settings `set`, `get` and `del`
- allow our custom test suite to run command-line tests
- clean-up Stream IO to allow us to print large things into debug stream (for example, `eeprom.dump`)
- send parsing errors to the debug log

As a proof of concept, introduce `TERMINAL_MQTT_SUPPORT` and `TERMINAL_WEB_API_SUPPORT`
- MQTT subscribes to the `<root>/cmd/set` and sends response to the `<root>/cmd`. We can't output too much, as we don't have any large-send API.
- Web API listens to the `/api/cmd?apikey=...&line=...` (or PUT, params inside the body). This one is intended as a possible replacement of the `API_SUPPORT`. Internals introduce a 'task' around the AsyncWebServerRequest object that will simulate what WiFiClient does and push data into it continuously, switching between CONT and SYS.

Both are experimental. We only accept a single command and not every command is updated to use Print `ctx.output` object. We are also somewhat limited by the Print / Stream overall, perhaps I am overestimating the usefulness of Arduino compatibility to such an extent :)
Web API handler can also sometimes show only part of the result, whenever the command tries to yield() by itself waiting for something. Perhaps we would need to create a custom request handler for that specific use-case.

code/espurna/debug.cpp

@@ -63,6 +63,11 @@ void _debugSend(const char * format, va_list args) {
 
 }
 
+void debugSendRaw(const char* line, bool timestamp) {
+    if (!_debug_enabled) return;
+    _debugSendInternal(line, timestamp);
+}
+
 void debugSend(const char* format, ...) {
 
     if (!_debug_enabled) return;
@@ -263,7 +268,7 @@ void debugSetup() {
 
     #if DEBUG_LOG_BUFFER_SUPPORT
 
-        terminalRegisterCommand(F("DEBUG.BUFFER"), [](Embedis* e) {
+        terminalRegisterCommand(F("DEBUG.BUFFER"), [](const terminal::CommandContext&) {
             _debug_log_buffer_enabled = false;
             if (!_debug_log_buffer.size()) {
                 DEBUG_MSG_P(PSTR("[DEBUG] Buffer is empty\n"));