For NewManager, the Config struct gets changed (not extended!) so that the
caller can provide a logger instead of just a logging function. Breaking the
API was chosen because it avoids having to maintain two different log calls in
various places (one for printf-style logging, one for structured logging).
RequestCertificateWithContext is an extension. It enables getting rid of
context.TODO calls.
NewFileStoreWithLogger also is an extension.
Kubernetes-commit: f9051901cee8d8ff4aed3db27ff495a706f1a487
certificate.FileStore only handles (cert, key) combined PEM files. This
PR allows (key, cert), which is what "openssl req -out foo.pem -keyout
foo.pem" generates.
Kubernetes-commit: 4b6a6a1cd5c8df83b3c51a03ecab975b82057489
