Add topic pub prototype completer

[artivis]

Add a message prototype completer to ros2 topic pub.
E.g.

$ ros2 topic pub /chatter geometry_msgs/msg/Pose [tab][tab]
-1
-n
--node-name
--once
-p
position:^J  x: 0.0^J  y: 0.0^J  z: 0.0^Jorientation:^J  x: 0.0^J  y: 0.0^J  z: 0.0^J  w: 0.0
--print
--qos-durability
--qos-profile
--qos-reliability
-r
--rate

Not very pretty but one can at least figure out the first few characters of the message body.
Then,

$ ros2 topic pub /chatter geometry_msgs/msg/Pose "p [tab]

resolves to,

ros2 topic pub /chatter geometry_msgs/msg/Pose "position:
  x: 0.0
  y: 0.0
  z: 0.0
orientation:
  x: 0.0
  y: 0.0
  z: 0.0
  w: 0.0"

This follows the work initiated in #298.

Signed-off-by: artivis jeremie.deray@canonical.com

[ivanpauno]

LGTM

[artivis]

@ivanpauno updated the PR to benefit from ros2/rosidl_python#78

[ivanpauno]

Sanity check:

• Linux:

[SinaMirrazavi]

This does not in Foxy apparently, or am I missing something?

[artivis]

@sinamr, I believe this is related to issue #563 and PR #570.