Implement token-authenticated registration (MSC3231)

[Givralix]

Pull Request Checklist

• I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• Pull request includes a sign off below using a legally identifiable name or I have already signed off privately

I will sign off privately.

TODO

• Add tests
• Add the /_matrix/client/v1/register/m.login.registration_token/validity endpoint
• Fix code style

[S7evinK]

Thanks for contributing to Dendrite!

The repo has been cloned into element-hq/dendrite and made the switch to AGPL as per https://matrix.org/blog/2023/11/06/future-of-synapse-dendrite/.
For more information, feel free to read this and this

If you still wish to contribute under the new license, feel free to create a new PR over at element-hq/dendrite.