refactor: rename exit_lidarparse_timestamp to exit_source_timestamp

[ZiningLi]

Description

This PR renames the exit_lidarparse_timestamp field to exit_source_timestamp in the LidarMeta structure to better reflect its generic nature as a source exit timestamp rather than being specific to the g3dlidarparse element.

Changes include

• Updated LidarMeta struct definition in g3d_lidar_meta.h
• Updated add_lidar_meta() function signature and implementation
• Updated JSON converter to output new field name exit_source_timestamp
• Updated g3dlidarparse element implementation
• Updated documentation in g3dinference.md
• Updated unit tests in gvametaconvert.cpp

Checklist:

• I agree to use the MIT license for my code changes.
• I have not introduced any 3rd party components incompatible with MIT.
• I have not included any company confidential information, trade secret, password or security token.
• I have performed a self-review of my code.