Funnels - API for getting persons on a step or drop-off

[paolodamico]

Issue to document the task of creating the API that will retrieve the person list at each step or drop-off in a funnel.

@buwilliams feel free to take over this issue.

[paolodamico]

Full product specs can be found here, but an important consideration is that we clearly show in the interface whether a session recording is available for each specific person. I'm thinking this information might need to come from the API but I'll let you suggest the best approach here. CC @mariusandra