[intel-npu] Adding EXECUTION_DEVICES read-only property (openvinotoolkit#24531)

### Details:
 - Adding EXECUTION_DEVICES read-only property for intel-npu plugin.

### Tickets:
 - EISW-112990

Author: csoka

src/plugins/intel_npu/src/plugin/src/plugin.cpp

@@ -364,6 +364,16 @@ Plugin::Plugin()
           [&](const Config& config) {
               return _metrics->GetDeviceType(get_specified_device_name(config));
           }}},
+        {ov::execution_devices.name(),
+         {true,
+          ov::PropertyMutability::RO,
+          [&](const Config& config) {
+              if (_metrics->GetAvailableDevicesNames().size() > 1) {
+                  return std::string("NPU." + config.get<DEVICE_ID>());
+              } else {
+                  return std::string("NPU");
+              }
+          }}},
         // OV Internals
         // =========
         {ov::internal::caching_properties.name(),