add 'enabled' parameter to set dynamically

[dujeong]

There was already a service that had a function similar to toggle stvl. (service name: <observation_topic>/toggle_enabled) The service can be set up for each individual topic, but I wanted a toggle function for the entire stvl.

So I implemented the function and made it dynamically configurable.

I referred to the functions(activate, deactivate).

[dujeong]

@SteveMacenski I added checking nullptr part in the code. Is that what you meant?

[dujeong]

Sure! I tested it on the robot. It worked well. :)

[SteveMacenski]

Thanks for the contribution!