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/platformio.ini

@@ -32,6 +32,11 @@ build_flags =
   -D PIN_TFT_LEDA_CTL=21     ; LEDK (switches on/off via mosfet to create the ground)
   -D PIN_GPS_RX=33
   -D PIN_GPS_TX=34
+  -D PIN_GPS_EN=35              ; N-ch MOSFET Q2 drives P-ch high-side switch → active HIGH (default)
+  -D PIN_GPS_RESET=36
+  -D PIN_GPS_RESET_ACTIVE=LOW
+  -D GPS_BAUD_RATE=115200
+  -D ENV_INCLUDE_GPS=1
   -D SX126X_DIO2_AS_RF_SWITCH=true
   -D SX126X_DIO3_TCXO_VOLTAGE=1.8
   -D SX126X_CURRENT_LIMIT=140