This adds an API for third-party plugin modules to implement account validity, so they can provide this feature instead of Synapse. The module implementing the current behaviour for this feature can be found at https://github.com/matrix-org/synapse-email-account-validity.
To allow for a smooth transition between the current feature and the new module, hooks have been added to the existing account validity endpoints to allow their behaviours to be overridden by a module.
