feat(refactor): Restructure project for improved modularity and clarity

This commit introduces a major refactoring of the project structure to align
with Zephyr's recommended multi-application and library organization.

Key changes include:
- Relocation of custom modules from 'software/modules/' to 'software/lib/'.
- Introduction of a central 'software/CMakeLists.txt' to manage application
  and library subdirectories.
- Creation of new Kconfig files for 'software/' and 'software/apps/slave_node/'
  to define project-wide and application-specific configurations.
- Removal of the 'gateway' and 'stm32g431_tests' applications.
- Removal of 'shell_modbus.c' and 'shell_system.c' from 'slave_node' application's
  direct source files, indicating a shift towards library-based shell commands.
- Updates to 'software/apps/slave_node/CMakeLists.txt', 'prj.conf', and
  'boards/bluepill_f103rb.conf' to reflect the new structure and dependencies.

software/include/lib/modbus_server.h

@@ -0,0 +1,52 @@
+#ifndef MODBUS_SERVER_H
+#define MODBUS_SERVER_H
+
+#include <stdint.h>
+
+/**
+ * @brief Modbus Input Register Addresses.
+ */
+enum {
+	/* Valve Control & Status */
+	REG_INPUT_VALVE_STATE_MOVEMENT        = 0x0000,
+	REG_INPUT_MOTOR_CURRENT_MA            = 0x0001,
+	/* Digital Inputs */
+	REG_INPUT_DIGITAL_INPUTS_STATE        = 0x0020,
+	REG_INPUT_BUTTON_EVENTS               = 0x0021,
+	/* System Config & Status */
+	REG_INPUT_FIRMWARE_VERSION_MAJOR_MINOR = 0x00F0,
+	REG_INPUT_FIRMWARE_VERSION_PATCH      = 0x00F1,
+	REG_INPUT_DEVICE_STATUS               = 0x00F2,
+	REG_INPUT_UPTIME_SECONDS_LOW          = 0x00F3,
+	REG_INPUT_UPTIME_SECONDS_HIGH         = 0x00F4,
+	/* Firmware Update */
+	REG_INPUT_FWU_LAST_CHUNK_CRC          = 0x0100,
+};
+
+/**
+ * @brief Modbus Holding Register Addresses.
+ */
+enum {
+	/* Valve Control */
+	REG_HOLDING_VALVE_COMMAND             = 0x0000,
+	REG_HOLDING_MAX_OPENING_TIME_S        = 0x0001,
+	REG_HOLDING_MAX_CLOSING_TIME_S        = 0x0002,
+	/* Digital Outputs */
+	REG_HOLDING_DIGITAL_OUTPUTS_STATE     = 0x0010,
+	/* System Config */
+	REG_HOLDING_WATCHDOG_TIMEOUT_S        = 0x00F0,
+	REG_HOLDING_DEVICE_RESET              = 0x00F1,
+	/* Firmware Update */
+	REG_HOLDING_FWU_COMMAND               = 0x0100,
+	REG_HOLDING_FWU_CHUNK_OFFSET_LOW      = 0x0101,
+	REG_HOLDING_FWU_CHUNK_OFFSET_HIGH     = 0x0102,
+	REG_HOLDING_FWU_CHUNK_SIZE            = 0x0103,
+	REG_HOLDING_FWU_DATA_BUFFER           = 0x0180,
+};
+
+int modbus_server_init(void);
+int modbus_reconfigure(uint32_t baudrate, uint8_t unit_id);
+uint32_t modbus_get_baudrate(void);
+uint8_t modbus_get_unit_id(void);
+
+#endif // MODBUS_SERVER_H