Add ACME-based certificate provider / addon

[Kaliumhexacyanoferrat]

As an operator of a web application using GenHTTP, I want the server to automatically fetch and update certificates via the ACME protocol, so I do not need to care about that process.

Example

var certificateManager = CertificateManager.LetsEncrypt();

await Host.Create()
          .Add(certificateManager);

Acceptance criteria

• The functionality is provided in a new module
• There is a one-line integration that binds to the typical ports (80, 443)
• The certificate manager automatically issues and updates certificates (both on start and during operation)
• During an update of the certificate, there is no application downtime
• The .well-known handling transparently allows other .well-known requests handled by the app
• The manager works with any ACME server
• The manager logs events when certificates are changed
• The manager stores private keys in a secure way (chmod etc.)
• The feature works with all engines (?)
• The feature is documented on the GenHTTP website
• The feature is covered by acceptance tests