Create start/pause/resume/stop sub-commands for 'ros2 trace'

[christophebedard]

Closes #68

That adds the following sub-commands to ros2 trace as "verbs":

• ros2 trace start: configure tracing session and start tracing
• ros2 trace pause: stop tracing, without removing tracing session
• ros2 trace resume: start tracing again
• ros2 trace stop: stop tracing and destroy tracing session

The goal is to provide non-interactive & finer-grained ways to control tracing. For example, this could be used in scripts. It could also be used for long-term tracing (related to #44): users can trace the first 10-30 seconds to collect initialization information, then pause, and then resume tracing again later on.

Example:

$ ros2 trace start my_session
UST tracing enabled (35 events)
kernel tracing disabled
context (3 fields)
writing tracing session to: /home/user/.ros/tracing/my_session
$ ros2 trace pause my_session
$ ros2 trace resume my_session
$ ros2 trace stop my_session

[christophebedard]

@mjcarroll friendly ping

[mjcarroll]

LGTM, just one copyright nit

[christophebedard]

• Linux
• Linux-aarch64
• Windows

[christophebedard]

Thanks for the review.

CI looks good, so let's go.