Introduce a separate step-finish RPC

[wojcik91]

Initially we decided to reuse the finish endpoint for the new multi-step MFA flow.
This complicates the logic, because for each request we have to decide if we are dealing with a legacy client finishing MFA or with a new client just finishing a step.
A cleaner approach would be to separate the clients at RPC level.

[wojcik91]

I'd also separate the mobile-approve endpoint from normal finish.
In general I think the whole legacy flow should target a separate flow in the MFA engine. The multi-purpose methods we inherited from previous MFA implementation lead to unnecessary edge-cases.