Add subprocess run function that duplicates streams
Add subprocess run function that duplicates streams

Add run-like function to in_toto.process module that redirects
standard output and standard error of executed command to temporary
files and both writes the contents to the corresponding streams of
the parent process during execution and also returns them along
with the child process's return code upon its termination.

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib