Add NimBLEAttValue class. (#67)

This is a specialized container class to hold BLE attribute values.

- Removes the use of std::string previously used to store the values.
- Allows for setting/getting/notifying values from std::string, std::vector<uint8_t>, Arduino String, const char*, and uint8_t buffers.
- Has operators retrieve the value as std::string, Arduino String, std::vector<uint8_t>, uint8_t* or char pointers.
- Includes iterators and subscript/random access operator.
- Introduces a max length parameter to the creation of server characteristics/descriptors to limit the size of the memory footprint.
- Nearly Seamless integration with existing code.
- Adds a config option to enable/disable timestamp storage when the value is updated.
- Adds a config option to specify the initial size of the value container if not specified in the constructor.

src/NimBLEClient.h

@@ -21,6 +21,7 @@
 #include "NimBLEUUID.h"
 #include "NimBLEUtils.h"
 #include "NimBLEConnInfo.h"
+#include "NimBLEAttValue.h"
 #include "NimBLEAdvertisedDevice.h"
 #include "NimBLERemoteService.h"
 
@@ -51,9 +52,9 @@ public:
     NimBLERemoteService*                        getService(const NimBLEUUID &uuid);
     void                                        deleteServices();
     size_t                                      deleteService(const NimBLEUUID &uuid);
-    std::string                                 getValue(const NimBLEUUID &serviceUUID, const NimBLEUUID &characteristicUUID);
+    NimBLEAttValue                              getValue(const NimBLEUUID &serviceUUID, const NimBLEUUID &characteristicUUID);
     bool                                        setValue(const NimBLEUUID &serviceUUID, const NimBLEUUID &characteristicUUID,
-                                                         const std::string &value, bool response = false);
+                                                         const NimBLEAttValue &value, bool response = false);
     NimBLERemoteCharacteristic*                 getCharacteristic(const uint16_t handle);
     bool                                        isConnected();
     void                                        setClientCallbacks(NimBLEClientCallbacks *pClientCallbacks,