net: tls_credentials: credential_digest

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>