ModuleAPI: provide method to set email / threepid

This issue has been migrated from [#13402](https://github.com/matrix-org/synapse/issues/13402).

---

I am comparing / syncing user email on login, and for this i currently use:

```python
async def set_email(
self,
user_id: str,
email: str
) -> bool:
threepid_dict_list = await self.api.get_threepids_for_user(user_id)
for threepid_dict in threepid_dict_list:
if threepid_dict['address'] == email:
logger.info('User %s already has this email set. Nice', user_id)
return False
logger.info('User %s did not have email %s set, adding it', user_id, email)
current_time = self.api._hs.get_clock().time_msec()
threepid_dict = {
"medium": "email",
"address": email,
"validated_at": current_time,
}
registration_handler = self.api._hs.get_registration_handler()
await registration_handler._register_email_threepid(user_id, threepid_dict, None)
return True
```

Which isn't great, because i go through api._hs.
Would be nice to have this provided by the API itself.