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.cpp

@@ -240,6 +240,24 @@ uint16_t NimBLERemoteCharacteristic::getDefHandle() {
 } // getDefHandle
 
 
+/**
+ * @brief Get iterator to the beginning of the vector of remote descriptor pointers.
+ * @return An iterator to the beginning of the vector of remote descriptor pointers.
+ */
+std::vector<NimBLERemoteDescriptor*>::iterator NimBLERemoteCharacteristic::begin() {
+    return m_descriptorVector.begin();
+}
+
+
+/**
+ * @brief Get iterator to the end of the vector of remote descriptor pointers.
+ * @return An iterator to the end of the vector of remote descriptor pointers.
+ */
+std::vector<NimBLERemoteDescriptor*>::iterator NimBLERemoteCharacteristic::end() {
+    return m_descriptorVector.end();
+}
+
+
 /**
  * @brief Get the descriptor instance with the given UUID that belongs to this characteristic.
  * @param [in] uuid The UUID of the descriptor to find.