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.cpp

@@ -83,3 +83,21 @@ void HeltecV4Board::begin() {
     return loRaFEMControl.getFEMType() == KCT8103L_PA ? "Heltec V4.3 OLED" : "Heltec V4 OLED";
 #endif
   }
+
+  bool HeltecV4Board::setLoRaFemLnaEnabled(bool enable) {
+    if (!loRaFEMControl.isLnaCanControl()) {
+      return false;
+    }
+
+    loRaFEMControl.setLNAEnable(enable);
+    loRaFEMControl.setRxModeEnable();
+    return true;
+  }
+
+  bool HeltecV4Board::canControlLoRaFemLna() const {
+    return loRaFEMControl.isLnaCanControl();
+  }
+
+  bool HeltecV4Board::isLoRaFemLnaEnabled() const {
+    return loRaFEMControl.isLNAEnabled();
+  }