Allow adding a "reason" for re-auth flow

**Problem**
Right now, when a certain action trigger a re-auth flow, users will be redirected immediately to the *Enter password* screen. Which is confusing for users.

For example, when you download the Admin API keys in Authgear Portal:


You will be redirected to this page immediately:

It is confusing, as users do not know why they have to type the password, it feel even like a phishing page if you're supicious.

**Appetite**
1 or 2 weeks

**Solution**
One idea is whenever a re-auth flow might be trigger, we shall have a "reason" string as a parameter, and show that reason at a prominent area in the Enter Password screen.

**Rabbit holes**
Details about the solution worth calling out to avoid problems

**No-goes**
Functionality or use cases we intentionally aren’t covering to fit the appetite

Start by tracing the re-authentication flow from the action trigger to the Enter Password screen, then identify the parameter and UI paths involved. Done means callers can provide a reason and users see it prominently before entering their password, with the affected flow covered by tests.

Written by the indexing model from the issue text.