gsi_util: adding pull subcommand

'pull' command can pull a file from an image file, folder or adb.

The patch includes a "mounter" framework to implement different
source of the system/vendor image. And also includes several
"mounter" implementations.

CompositeMounter integrates all possible mounter implementations.
Usually just using CompositeMounter is enough. With
CompositeMounter, you could access files in different target
with an unique interface, such files in an image file, a folder or
a device with an unique interface. pull.py is an basic example to
use CompositeMounter.

Here are some example to use 'pull' command:

$ ./gsi_util.py pull --system adb:AB0123456789 /system/manifest.xml
$ ./gsi_util.py pull --vendor adb /vendor/compatibility_matrix.xml
$ ./gsi_util.py pull --system system.img /system/build.prop
$ ./gsi_util.py pull --system my/out/folder/system /system/build.prop

As current implementation, accessing files in a the image file requires
root permission. gsi_util will need user to input the password for sudo.

For the detail usage, reference:

$ ./gsi_util.py pull --help

Bug: 71029338
Test: pull /system/build.prop from different targets
Change-Id: Iaeb6352c14ebc24860ed79fc30edd314e225aef9

gsi/gsi_util/gsi_util.py

@@ -28,7 +28,7 @@ class GsiUtil(object):
 
   # Adds gsi_util COMMAND here.
   # TODO(bowgotsai): auto collect from gsi_util/commands/*.py
-  _COMMANDS = ['flash_gsi', 'hello']
+  _COMMANDS = ['flash_gsi', 'pull', 'hello']
 
   _LOGGING_FORMAT = '%(message)s'
   _LOGGING_LEVEL = logging.WARNING