`process.GetInfoForPid` collects the total thread count

What does this PR do?

Collect the total thread count, num_threads, as part of the information collected by process.GetInfoForPid

On linux:
- it extends the information parsed from /proc/[PID]/stat to get the total thread count
- unit tests added, they mock /proc/[PID]/stat
On darwin:
- it uses the syscall proc_pidinfo
- unit test only validates GetInfoForPid does not return an error and the total thread count ins't zero
On windows:
- it uses the Process Snapshotting APIs: https://learn.microsoft.com/en-us/previous-versions/windows/desktop/proc_snap/overview-of-process-snapshotting
- unit test only validates GetInfoForPid does not return an error and the total thread count ins't
Add a unit test using a C++ program which runs exactly 42 threads so we can assert the data read by process.GetInfoForPid On windows the and newer the process start with more threads than the program requests to parallel load needed tabels/dll or some windows specific things. Therefore the test on windows accepts 42 or 45 for the number of threads allocated to the process. See the following for details https://stackoverflow.com/questions/42789199/why-there-are-three-unexpected-worker-threads-when-a-win32-console-application-s/42789684#42789684

To enable metricbeat to collect the total thread count per process as requested by https://github.com/elastic/beats/issues/34461.