Add support for a Web Key Service

[Tobi Nary]

Web Key Service allows automatic, standardized publication of Open-PGP public keys to a Web Key Directory.

Web Key Directory (WKD) is a service that allows easy, decentralized and standardized key discovery for Open-PGP-recipients' public keys while making a fair amount of initial trust available.

Public keys of email users of provider A are hosted at https://a.tld/.well-known/openpgpkey/hu/someuserslocalhash. Email clients can now probe that url for a public key and use it for encryption. Since the server is authenticated by its TLS certificate, an initial trust is established for that key.

Web Key Service (WKS) allows for automation of the publishing of keys to a WKD. Mail clients of users of provider A may use a (standardized) way to publish new or changed keys to the providers WKD, for example by sending an OpenPGP/MIME E-mail using the authentication method established between the user and A.

The provider can then easily verify that the email is indeed authenticated and publish the key in it's WKD. This process can be automated.

Having this option for the mail server would be great. Yet, I cannot find the right hooks to implement such a feature as a plugin. Additionally, I think easy and automatic WKS/WKD support out of the box would be a good sales pitch for this project. At least for LE-configured servers, this should be easy to implement and offer a great improvement.