refactor: Improve bctoolboxTimeSpec handling by adding the bctbx_timespec_add_secs() function

Refactor time handling in the bctoolbox library by renaming the existing bctbx_timespec_add function to bctbx_timespec_add_secs and introducing a new bctbx_timespec_add_milliseconds function, to be able to add milliseconds to a timespec. Adjust the related tests to reflect these changes.