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/fwu.h

@@ -0,0 +1,10 @@
+#ifndef FWU_H
+#define FWU_H
+
+#include <stdint.h>
+
+void fwu_init(void);
+void fwu_handler(uint16_t addr, uint16_t reg);
+uint16_t fwu_get_last_chunk_crc(void);
+
+#endif // FWU_H