Add a module API callback to override whether a given user is within the MAU trial period.

This issue has been migrated from [#12502](https://github.com/matrix-org/synapse/issues/12502).

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[`is_trial_user`](https://github.com/matrix-org/synapse/blob/8af8a9bce5319ebf99573433f9f89d94767e173c/synapse/storage/databases/main/registration.py#L216-L231) is used to determine whether a given user is within the "trial period" for MAU. This is a grace period that means users who are active for less than N days are not counted as active users as far as MAU is concerned.

We've got a use case where specific users (as determined by their registration origin) will have different trial periods, so one way of achieving this would be a module callback to determine this.

Start in synapse/storage/databases/main/registration.py at is_trial_user, which currently determines whether a user is within the MAU trial period. Trace how that result is used and review the module API integration points before defining the callback behavior. Done means module-provided logic can determine the trial period for a user based on registration origin.

Written by the indexing model from the issue text.