core(fr): update flow API for 10.0

[adamraine]

Closes #14049

• Only one config can be used for the entire user flow. Individual steps can specify settings overrides via the step flags but cannot use an entirely different config.
• Step runners use page as a top level parameter rather than stuffing it into {page, config, flags}
• New StepFlags extends LH.Flags which contains the step name instead of doing {stepName, flags}
• Rename stepName -> name

[adamraine]

TODO: Docs need updating

Rest of this should be reviewable though.