Rework backend add MQTT and WebSocket support

* Update back end to add MQTT and WebSocket support
* Update demo project to demonstrate MQTT and WebSockets
* Update documentation to describe newly added and modified functionallity
* Introduce separate MQTT pub/sub, HTTP get/post and WebSocket rx/tx classes
* Significant reanaming - more accurate class names
* Use PROGMEM_WWW as default
* Update README documenting PROGMEM_WWW as default
* Update README with API changes

lib/framework/FSPersistence.h

@@ -0,0 +1,103 @@
+#ifndef FSPersistence_h
+#define FSPersistence_h
+
+#include <StatefulService.h>
+#include <JsonSerializer.h>
+#include <JsonDeserializer.h>
+#include <FS.h>
+
+#define MAX_FILE_SIZE 1024
+
+template <class T>
+class FSPersistence {
+ public:
+  FSPersistence(JsonSerializer<T> jsonSerializer,
+                JsonDeserializer<T> jsonDeserializer,
+                StatefulService<T>* statefulService,
+                FS* fs,
+                char const* filePath) :
+      _jsonSerializer(jsonSerializer),
+      _jsonDeserializer(jsonDeserializer),
+      _statefulService(statefulService),
+      _fs(fs),
+      _filePath(filePath) {
+    enableUpdateHandler();
+  }
+
+  void readFromFS() {
+    File settingsFile = _fs->open(_filePath, "r");
+
+    if (settingsFile) {
+      if (settingsFile.size() <= MAX_FILE_SIZE) {
+        DynamicJsonDocument jsonDocument = DynamicJsonDocument(MAX_FILE_SIZE);
+        DeserializationError error = deserializeJson(jsonDocument, settingsFile);
+        if (error == DeserializationError::Ok && jsonDocument.is<JsonObject>()) {
+          updateSettings(jsonDocument.as<JsonObject>());
+          settingsFile.close();
+          return;
+        }
+      }
+      settingsFile.close();
+    }
+
+    // If we reach here we have not been successful in loading the config,
+    // hard-coded emergency defaults are now applied.
+    applyDefaults();
+  }
+
+  bool writeToFS() {
+    // create and populate a new json object
+    DynamicJsonDocument jsonDocument = DynamicJsonDocument(MAX_FILE_SIZE);
+    JsonObject jsonObject = jsonDocument.to<JsonObject>();
+    _statefulService->read(jsonObject, _jsonSerializer);
+
+    // serialize it to filesystem
+    File settingsFile = _fs->open(_filePath, "w");
+
+    // failed to open file, return false
+    if (!settingsFile) {
+      return false;
+    }
+
+    // serialize the data to the file
+    serializeJson(jsonDocument, settingsFile);
+    settingsFile.close();
+    return true;
+  }
+
+  void disableUpdateHandler() {
+    if (_updateHandlerId) {
+      _statefulService->removeUpdateHandler(_updateHandlerId);
+      _updateHandlerId = 0;
+    }
+  }
+
+  void enableUpdateHandler() {
+    if (!_updateHandlerId) {
+      _updateHandlerId = _statefulService->addUpdateHandler([&](String originId) { writeToFS(); });
+    }
+  }
+
+ private:
+  JsonSerializer<T> _jsonSerializer;
+  JsonDeserializer<T> _jsonDeserializer;
+  StatefulService<T>* _statefulService;
+  FS* _fs;
+  char const* _filePath;
+  update_handler_id_t _updateHandlerId = 0;
+
+  // update the settings, but do not call propogate
+  void updateSettings(JsonObject root) {
+    _statefulService->updateWithoutPropagation(root, _jsonDeserializer);
+  }
+
+ protected:
+  // We assume the deserializer supplies sensible defaults if an empty object
+  // is supplied, this virtual function allows that to be changed.
+  virtual void applyDefaults() {
+    DynamicJsonDocument jsonDocument = DynamicJsonDocument(MAX_FILE_SIZE);
+    updateSettings(jsonDocument.to<JsonObject>());
+  }
+};
+
+#endif  // end FSPersistence