flows: add the ability to choose to wait or not for steps

[twittemb]

Description

This PR gives the ability to wait or not for flows readiness to execute a completion block.

Checklist

• [x] this PR is based on develop or a 'develop related' branch
• [x] the commits inside this PR have explicit commit messages
• [ ] the Jazzy documentation has been generated (if needed -> Jazzy RxFlow)