feat: add time into the telemetry payload

[zhongys-c8y]

Another option to add time is to always add a time fragment into the telemetry payloads.
This is the alternative option for this PR: #46

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
22	0	0	22	100	44.691024999s

Passed Tests

Name	⏱️ Duration	Suite
Device should support the operation c8y_ModbusConfiguration	0.183 s	Device
Set values via c8y_ModbusConfiguration Operation	2.570 s	Operation
Poll rate and transmit rate should be updated for the Device	0.406 s	Operation
Poll rate and transmit rate should be updated on the Device	2.572 s	Operation
Device should support the operation c8y_ModbusDevice	0.182 s	Device
Device should support the operation c8y_SerialConfiguration	0.259 s	Device
Set values via c8y_SerialConfiguration Operation	2.560 s	Operation
Serial configuration should be updated for the Device	0.346 s	Operation
Serial configuration should be updated on the Device	2.564 s	Operation
Device should have installed software tedge-modbus-plugin	0.206 s	Debian
Service should be active	2.561 s	Debian
ReInstall Modbus Plugin	22.446 s	Debian
Device should have the fragment c8y_ModbusConfiguration	0.363 s	Device
ChildDevice TestCase1 should be created	0.357 s	Device
ChildDevice TestCase1 should have the fragment c8y_ModbusDevice	0.884 s	Device
Service tedge-modbus-plugin should be enabled	0.182 s	Device
Device should set supported operations for writing to registers and coils	0.177 s	Operations
Device should write to a registers	2.580 s	Operations
Device should write to a coil	2.577 s	Operations
ChildDevice TestCase1 should have a Test.Int16 Measurement	0.358 s	Telemetry
ChildDevice TestCase1 should have a Test.Float32 Measurement	0.358 s	Telemetry
ChildDevice TestCase1 should have Alarms of type TestAlarm on Coil Value 1	0.001 s	Telemetry

[zhongys-c8y]

This approach should be the correct way to go rather than #46 . It will ignore messages on /cmd/ topic and won't change the payload if there is already a timestamp included