Add tasks to stop/start local DAST example servers

What does this MR do?

Makes it easy for DAST engineers to start and stop example servers on their local machine. This is useful to local exploratory testing.

New invoke task list:

dast $ invoke -l
Available tasks:

  server.ajaxspider
  server.basicsite
  server.restapi
  server.stopall
  zap.metadata

Example starting a server:

dast $ invoke server.restapi
ce0c02a5419ee306d6f788932f264b9c2e1bebbc82e1e5151719c585b04be727
REST API server started at http://localhost:8030/

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer