Implement more Zephyr MCUmgr Groups

[Finomnis]

We should strive towards being a universal Zephyr MCUmgr tool rather than just a firmware updater.

To achieve this, we should implement the following missing subsystems:

• Default/OS Management Group
• Application/software image management group
• Statistics management
• Settings (Config) Management Group
• File management
• Shell management
• Enumeration Management Group
• Zephyr Management Group