Allow timer rate to be configurable in`message_lost_talker`

[Yadunund]

By default the talker publishes an 8mB message every 3 seconds. I haven't been able to reproduce any message lost callback triggers with such a low frequency. It helps to increase the publishing rate. This PR

• Adds support to pass a -r CLI arg to message_lost_talker to configure the timer's period. Default behavior is still the same 3s period.
• Sets sub's reliability QoS to best_effort to increase the probability of dropping a message. -> I can undo this change if needed.

[sloretz]

@Yadunund friendly ping 🧇

[Yadunund]

Sorry for the delay. Will have this cleaned up by end of this week

[ahcorde]

@Yadunund I applied the suggested changes. Do you mind to take a look ?