Revise usage of Milestones in authflow

Some milestones, such as MilestoneDoCreateIdentity, are used to track steps or determine whether an action have been done or not inside an intent. However, when there are nested steps inside the intent, milestones in nested steps could affect the parent and causing bugs.

Therefore, we should see if we can restructure the nodes such that intents are only finding milestones in the current flow, and do not nest into subflows.

Start by tracing how authflow intents find milestones in the current flow and how nested steps create subflows. Compare MilestoneDoCreateIdentity with the parent and nested intent behavior, then determine the intended restructuring so parent intents ignore milestones from subflows. Done means nested milestones no longer affect parent milestone checks and the related authflow behavior is covered by tests.

Written by the indexing model from the issue text.