This new method collects all of the various different settings that
describe the operator's chosen OCI credentials policy and returns a single
object that encapsulates that policy.
This is the method that will, in future commits, be used by package main
to provide the credentials policy to any OCI-registry-related subsystems
using dependency-inversion style.
Signed-off-by: Martin Atkins <mart@degeneration.co.uk>
