[Feature request] Add APIs to pause/resume/start updaters #226

As I observed, there isn't any APIs to pause/resume/start diagnostic updaters. Especially when invoking diagnostic_updaters in LifecycleNodes, diagnostic updaters begins publishing topic to /diagnostics just after LifecycleNodes constructing. As description in Managed Nodes, publishers in Lifecyclenodes should be configured and activated before publishing topics.

In my opinion, there are two ways:

1. We need to add some APIs to pause/resume/start updaters for special usage. Then we will face with choices: Will updater run timer by default? or updater only run timer after called "start" function?
2. We add a new header file, eg. diagnostic_updater_lifecycle.hpp. And create a class derived from DiagnosticTaskVector that class Updater derived. Then implement all LifecycleNodes' APIs in it.