gst-launch: Add support printing current position of pipeline

Add new opetion "position" to enable print out current position.
Without this, users couldn't estimate how long the task will take.
8 jobs for launch-pos in 25 minutes and 9 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #163691
gst indent

00:02:06

passed #163690
manifest

00:02:19

 
  Build
passed #163692
build fedora x86_64

00:09:17

 
  Test
passed #163693
check fedora

00:04:00

 
  Full Builds
passed #163694
build android arm64 api28

00:04:58

passed #163696
build cerbero cross win32

00:09:31

passed #163697
build cerbero cross win64

00:08:01

passed #163695
build cerbero fedora x86_64

00:08:54