Add GPS support Heltec Wireless Tracker v1.x

Pin mapping verified against HTIT-Tracker V0.5 schematic:
- GPIO35 (GPS_EN): N-ch MOSFET drives P-ch high-side switch, active HIGH
- GPIO36 (GPS_RST): hardware reset, active LOW
- GPIO33/34: UART TX/RX

Delegates power management to MicroNMEALocationProvider begin()/stop()
which independently controls GPS power via GPS_EN and shares VEXT with
the display through RefCountedDigitalPin.

variants/heltec_tracker/target.h

@@ -28,6 +28,7 @@ public:
   const char* getSettingName(int i) const override;
   const char* getSettingValue(int i) const override;
   bool setSettingValue(const char* name, const char* value) override;
+  LocationProvider* getLocationProvider() override { return _location; }
 };
 
 extern HeltecV3Board board;