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.

docs/cli_commands.md

@@ -263,6 +263,20 @@ This document provides an overview of CLI commands that can be sent to MeshCore
 
 ---
 
+#### View or change the LoRa FEM receive-path gain state on supported boards
+**Usage:**
+- `get radio.fem.rxgain`
+- `set radio.fem.rxgain <state>`
+
+**Parameters:**
+- `state`: `on`|`off`
+
+**Notes:**
+- This controls the external LoRa FEM receive-path LNA where the board supports it.
+- This is separate from `radio.rxgain`, which controls the radio chip receive gain mode.
+
+---
+
 ### System
 
 #### View or change this node's name