Implement token-authenticated registration

*This issue was originally created by [**@S7evinK**](https://github.com/S7evinK) at .*

Spec version 1.2 introduced token-authenticated registration and a new client API endpoint (MSC3231), this is currently missing.

`GET /_matrix/client/v1/register/m.login.registration_token/validity` [spec](https://spec.matrix.org/v1.2/client-server-api/#get_matrixclientv1registermloginregistration_tokenvalidity)

To generate tokens, we should use the same [admin endpoints](https://github.com/matrix-org/synapse/blob/develop/synapse/rest/admin/registration_tokens.py) as Synapse, so existing tools to create tokens can be used for Dendrite as well.