Add LoRa FEM RX-path LNA control via CLI; restore enabled default

Ported from meshcore-dev/MeshCore#2140 (Quency-D <hj_zzns@163.com>).

The upstream v1.16.0 merge changed the Heltec FEM LNA to disabled by
default (upstream commit 696aae6e).  This patch restores the enabled
default for repeater via the new "radio.fem.rxgain" preference
(defaulted to 1), while also making the setting user-configurable
and persisted across reboots.

variants/heltec_v4/HeltecV4Board.h

@@ -25,6 +25,9 @@ public:
   void onAfterTransmit(void) override;
   void enterDeepSleep(uint32_t secs, int pin_wake_btn = -1);
   void powerOff() override;
+  bool setLoRaFemLnaEnabled(bool enable) override;
+  bool canControlLoRaFemLna() const override;
+  bool isLoRaFemLnaEnabled() const override;
   uint16_t getBattMilliVolts() override;
   bool setAdcMultiplier(float multiplier) override {
     if (multiplier == 0.0f) {