Support SAN certificates

[tazjin]

First idea:

Let users put JSON arrays into annotations, e.g.:

acme/certificate: '["tazj.in", "www.tazj.in"]'

Once #2 is fixed this should be a separate field in the resource!

Implementation wise:

1. Challenge handling has to be split out & done for each domain with the same Authorization.
2. We can probably do 1 in a parallelStream()

Q:

1. Certificate naming becomes unclear! This ties into Re-evaluate naming of secrets #17