Add runtime provisioning for APN and device settings

[sensmonitor]

Goal

Allow firmware users to configure APN and device-specific settings without rebuilding the firmware.

Why

The current firmware is developer-friendly, but binary releases are not practical until APN/operator/device settings can be configured at runtime.

Scope

• Define provisioning approach.
• Support APN configuration.
• Support optional username/password.
• Consider SensMonitor endpoint/device token or registration flow.
• Document the workflow.

Notes

This is a prerequisite for future firmware binary release assets.