Add rosbag2_interfaces package with playback service definitions

[emersonknapp]

These .srv definitions will be used to control playback on the Player. Recording controls not yet defined, so none added.

Part of #696
Design at https://github.com/ros2/rosbag2/blob/master/docs/design/rosbag2_playback_time.md

[Karsten1987]

I'd vote to rename the project to rosbag2_srvs.
I don't mind too much about the actual services, but some of them can be replaced by std_srvs.

[Karsten1987]

it's all good. you can go ahead and merge as is. I just wanted to bring it up.

[emersonknapp]

it's all good. you can go ahead and merge as is. I just wanted to bring it up.

I'm just presenting where my thinking came from - I don't have strong opinions here, I am definitely open to making changes if there are good counterpoints.

For the package naming - I was thinking that potentially Play, Record, or Convert/Rewrite could be controlled by an Action with progress feedback - I don't have any Messages in mind right now though.

[Karsten1987]

I agree with your reasoning here. Thanks for the explanations.

[david-prody]

This looks good

[emersonknapp]

Gist: https://gist.githubusercontent.com/emersonknapp/20e6f6611bb120321b7294df2f6974bb/raw/9560335eef310dd4ebe284927edc903bb23f43a2/ros2.repos
BUILD args: --packages-up-to rosbag2 rosbag2_interfaces
TEST args: --packages-select rosbag2 rosbag2_interfaces
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/8207

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