Support for external key creation

[twenzel]

We're using the automatic key management/rotation.

We're now facing the problem that these created X509 Certificates should be signed/created from a public signing authority (e.g. Let's Encrypt).

Can please create an interface to be able to implement a custom logic to create the new keys. We still want to use the existing rotation logic with announcement and retire phase, just a custom logic to create the RSA/EC keys.

[ProgrammerAL]

@twenzel, I’ll definitely bring this up with the development team. But before I do that, can you share some information about your use case? Including the reason for needing a public signing authority.

[twenzel]

The key is also used for the dynamic SAML providers. A customer of us requires a SAML auth request with a signature of a public signed certificate.
Therefore we want to use Let's Encrypt to automatically create new certificates (using the ACME protocol).