Refactor NimBLEAttValue.

* Add length parameter to `setValue()` templates, with defaults for backward compatibility.
* Changed `setValue(const char*)` to add an optional length parameter so that a NULL character can be included, defaults to strlen.
* Moved non-inline functions to `NimBLEAttValue.cpp` file.
* Corrected includes to permit compilation as a stand-alone utility.
* Removed repeated code in `setValue()` by using `append()` after clearing the value.
* General code cleanup.

CMakeLists.txt

@@ -39,6 +39,7 @@ idf_component_register(
     "src/NimBLEAddress.cpp"
     "src/NimBLEAdvertisedDevice.cpp"
     "src/NimBLEAdvertising.cpp"
+    "src/NimBLEAttValue.cpp"
     "src/NimBLEBeacon.cpp"
     "src/NimBLECharacteristic.cpp"
     "src/NimBLEClient.cpp"