NFR: Simplify the timestamp handling modes by merging the PTP timestamp mode with the INTERNAL_OSC and PULSE_IN

[Samahu]

Is your feature request related to a problem? Please describe.
Mainly to simplify the code

Simplify the timestamp handling modes by merging the PTP mode with the INTERNAL_OSC and PULSE_IN

Describe the solution you'd like

We can merge the handling of PTP timestamp mode with the INTERNAL_OSC and PULSE_IN by using a default ptp utc timestamp to zero and then the user would be responsible to set the timestamp mode.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Targeted Platform (please complete the following information only if applicable, otherwise dot N/A):

• Ouster Sensor? ANY
• Ouster Firmware Version? ANY
• ROS version/distro? ANY
• Operating System? ANY
• Machine Architecture? ANY