client-go certificate: context-aware APIs and logging

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.

Author: pohly