wfe: Add IP address rendering check to prepAuthorizationForDisplay

[jprenken]

RFC 8738 places a restriction on the format in which we render IP address identifiers to ACME clients:

The value field of the identifier MUST contain the textual form of the address as defined in Section 2.1 of [RFC1123] for IPv4 and in Section 4 of [RFC5952] for IPv6.

We pass all identifiers received in new-order requests through policy.ValidIP, which ensures this.

However, we should augment this request-time enforcement with a render-time check inside wfe.prepAuthorizationForDisplay.