Turn on sample data generation with flag

There is now a list of integration packages available which are able to generate sample data, see https://github.com/elastic/elastic-package/pull/1634 for more details. It would be nice, if elastic-package could be started with a flag that automatically installs some of these integrations and starts the sample data generation so when testing features, data is already available and ingested in real time.

Start by reviewing pull request 1634 to understand the available integration packages and sample-data generation flow, then inspect the CLI startup and flag handling entry points. Done should mean a documented flag installs selected integrations and starts real-time sample data generation, but the issue does not define selection, defaults, or expected tests.

Written by the indexing model from the issue text.