This moves the encryption cipher parameter out of the WriteConfig()
method in the ConfigWriter interface and into a new ConfigWriterOptions
struct. Options (e.g. cipher) can be applied to an existing ConfigWriter
to allow customizing implementation-specific behavior.
This PR adds the ability to configure different backends to use for
storing modifications to the zero bootstrap config. The two currently
implemented backends allow writing changes to a file or to a Kubernetes
secret. Backend selection is determined by the scheme in a URI passed to
the flag '--config-writeback-uri'.
In a Kubernetes environment, where the bootstrap config is mounted into
the pod from a secret, this option allows Pomerium to write changes back
to the secret, as writes to the mounted secret file on disk are not
persisted.
