Add timeout to PrivContext and entrypoint_with_timeout decorator

entrypoint_with_timeout decorator can be used with a timeout parameter,
if the timeout is reached PrivsepTimeout is raised.
The PrivContext has timeout variable, which will be used for all
functions decorated with entrypoint, and PrivsepTimeout is raised if
timeout is reached.

Co-authored-by: Rodolfo Alonso <ralonsoh@redhat.com>
Change-Id: Ie3b1fc255c0c05fd5403b90ef49b954fe397fb77
Related-Bug: #1930401