Add an override to `one_hint_per_goal` in the hint distribution

[Elagatua]

Allows the one_hint_per_goal setting to be overridden when the value is explicitly defined in the hint distribution file. If the value is omitted, the flag will default to the existing behavior that sets it based on whether the goal categories contain the same goals.

Usage:

    "one_hint_per_goal":     true,

This code has been in use for the SGL settings since 2023, so it is relatively well tested.