Refactored some of the existing bits for testing the Authority
API by creation of a new LinkedAuthority interface and changing
visibility of the MockAuthority to be usable by other packages.
At this time, not all of the functions of MockAuthority it usable
yet. Will refactor when needed or requested.
Before this commit, EAB keys could be used CA-wide, meaning that
an EAB credential could be used at any ACME provisioner. This
commit changes that behavior, so that EAB credentials are now
intended to be used with a specific ACME provisioner. I think
that makes sense, because from the perspective of an ACME client
the provisioner is like a distinct CA.
Besides that this commit also includes the first tests for EAB.
The logic for creating the EAB JWS as a client has been taken
from github.com/mholt/acmez. This logic may be moved or otherwise
sourced (i.e. from a vendor) as soon as the step client also
(needs to) support(s) EAB with ACME.
