feat: Add Apple Silicon GPU support via IOReport API

Add GPU monitoring for Apple Silicon Macs using the IOReport framework
and IOHIDEventSystem for temperature readings.

Metrics supported:
- GPU utilization (from GPU Performance States residency)
- GPU power consumption (from Energy Model channel)
- GPU temperature (from IOHIDEventSystem thermal sensors)
- GPU clock speed (from DVFS frequency table + residency weighting)
- Memory usage (unified memory architecture, reports system RAM)

Implementation follows the existing Nvml/Rsmi/Intel backend pattern with
init/shutdown/collect<is_init> template functions in a new AppleSilicon
namespace under Gpu.

Build system changes:
- Enable GPU_SUPPORT on macOS in both CMakeLists.txt and Makefile
- Link IOReport library on macOS
- Add 'apple' to shown_gpus vendor filter

Closes #955

src/btop_config.cpp

@@ -239,7 +239,7 @@ namespace Config {
 		{"rsmi_measure_pcie_speeds",
 								"#* Measure PCIe throughput on AMD cards, may impact performance on certain cards."},
 		{"gpu_mirror_graph",	"#* Horizontally mirror the GPU graph."},
-		{"shown_gpus",			"#* Set which GPU vendors to show. Available values are \"nvidia amd intel\""},
+		{"shown_gpus",			"#* Set which GPU vendors to show. Available values are \"nvidia amd intel apple\""},
 		{"custom_gpu_name0",	"#* Custom gpu0 model name, empty string to disable."},
 		{"custom_gpu_name1",	"#* Custom gpu1 model name, empty string to disable."},
 		{"custom_gpu_name2",	"#* Custom gpu2 model name, empty string to disable."},
@@ -288,7 +288,7 @@ namespace Config {
 		{"custom_gpu_name4", ""},
 		{"custom_gpu_name5", ""},
 		{"show_gpu_info", "Auto"},
-		{"shown_gpus", "nvidia amd intel"}
+		{"shown_gpus", "nvidia amd intel apple"}
 	#endif
 	};
 	std::unordered_map<std::string_view, string> stringsTmp;