Add iterators to client remote attributes.

Add iterators for NimBLEScan: NimBLEadvertisedDevice, NimBLEClient: NimBLERemoteService, NimBLERemoteService: NimBLERemoteCharacteristic and NimBLERemoteCharacteristic: NimBLERemoteDescriptor

This is handy e.g. for showing every address of the advertised devices from a scan. To do so, first get a new scan and next:
```
for(auto pAdvertisedDevice: pBLEScan->getResults()) {
  Serial.printf("Address is %s\n", std::string(pAdvertisedDevice->getAddress()).c_str());
}
```
Of course any other property of the advertised device can be shown (or looked up, if that is your use case)

Also this is handy e.g. for showing every UUID in a peripheral. To do so, first connect to a peripheral and next:
```
for(auto pService: *pClient) {
  Serial.printf("Service UUID is %s\n", std::string(pService->getUUID()).c_str());
  for(auto pCharacteristic: *pService) {
    Serial.printf("Characteristic UUID is %s\n", std::string(pCharacteristic->getUUID()).c_str());
    for(auto pDescriptor: *pCharacteristic) {
      Serial.printf("Descriptor UUID is %s\n", std::string(pDescriptor->getUUID()).c_str());
    }
  }
}
```
Again of course any other property can be shown, or looked up.

src/NimBLERemoteCharacteristic.h

@@ -42,29 +42,31 @@ public:
     ~NimBLERemoteCharacteristic();
 
     // Public member functions
-    bool        canBroadcast();
-    bool        canIndicate();
-    bool        canNotify();
-    bool        canRead();
-    bool        canWrite();
-    bool        canWriteNoResponse();
-    NimBLERemoteDescriptor* getDescriptor(const NimBLEUUID &uuid);
-    std::vector<NimBLERemoteDescriptor*>* getDescriptors();
-    uint16_t    getHandle();
-    uint16_t    getDefHandle();
-    NimBLEUUID  getUUID();
-    std::string readValue();
-    uint8_t     readUInt8();
-    uint16_t    readUInt16();
-    uint32_t    readUInt32();
-    bool        registerForNotify(notify_callback _callback, bool notifications = true, bool response = true);
-    bool        writeValue(const uint8_t* data, size_t length, bool response = false);
-    bool        writeValue(const std::string &newValue, bool response = false);
-    bool        writeValue(uint8_t newValue, bool response = false);
-    std::string toString();
-    uint8_t*    readRawData();
-    size_t      getDataLength();
-    NimBLERemoteService* getRemoteService();
+    bool                                           canBroadcast();
+    bool                                           canIndicate();
+    bool                                           canNotify();
+    bool                                           canRead();
+    bool                                           canWrite();
+    bool                                           canWriteNoResponse();
+    std::vector<NimBLERemoteDescriptor*>::iterator begin();
+    std::vector<NimBLERemoteDescriptor*>::iterator end();
+    NimBLERemoteDescriptor*                        getDescriptor(const NimBLEUUID &uuid);
+    std::vector<NimBLERemoteDescriptor*>*          getDescriptors();
+    uint16_t                                       getHandle();
+    uint16_t                                       getDefHandle();
+    NimBLEUUID                                     getUUID();
+    std::string                                    readValue();
+    uint8_t                                        readUInt8();
+    uint16_t                                       readUInt16();
+    uint32_t                                       readUInt32();
+    bool                                           registerForNotify(notify_callback _callback, bool notifications = true, bool response = true);
+    bool                                           writeValue(const uint8_t* data, size_t length, bool response = false);
+    bool                                           writeValue(const std::string &newValue, bool response = false);
+    bool                                           writeValue(uint8_t newValue, bool response = false);
+    std::string                                    toString();
+    uint8_t*                                       readRawData();
+    size_t                                         getDataLength();
+    NimBLERemoteService*                           getRemoteService();
 
 private: