[PoC] Implement open funnels

[zoldar]

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[github-actions[bot]]

Preview environment👷🏼‍♀️🏗️
PR-5810

[wadjeroudi]

@zoldar nice PR, I see that it could close two feature requests: https://feedback.plausible.io/388 (Optional steps in funnel) https://feedback.plausible.io/420

does it handle custom property filter correctly ? for example if a step is optional but has a filter