Adds an internal credential_digest for generating a string digest of
credentials.
Such digests would allow users of a prospective TLS credentials shell to
verify the contents of a given credential without directly accessing
those contents.
Offloading the digest process to the underlying backend allows backends
for which private portions are not directly accessible to be eventually
supported.
Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>
