Build failure since galactic due to missing default_value for period parameter

[nlimpert]

rclcpp API changes between foxy and galactic enforce the default_value for declare_parameter:

• https://docs.ros2.org/foxy/api/rclcpp/classrclcpp_1_1Node.html#a095ea977b26e7464d9371efea5f36c42
• https://docs.ros2.org/galactic/api/rclcpp/classrclcpp_1_1Node.html

Hence starting with galactic the sw_watchdog package's simple_heartbeat node won't build as it does not set any default value.

I was going to fix this myself but saw that @jeskesen created a patch for this - @jeskesen would you mind creating a PR accordingly so we can discuss what's required to merge this?

[Hommus]

Keen to see this merged :+1:

[vinnnyr]

Coincidentally was blowing some dust off an old project and ran into the same thing. Implemented the same change as above and opened a PR.