Use DNSSEC for resolving domain names

[ethanjli]

Right now we're using trust-dns-resolver to look up DNS records for domain names, but we probably haven't enabled DNSSEC features yet. trust-dns-resolver states that enabling DNSSEC requires enabling the feature dnssec-openssl or dnssec-rustls (we should probably use the latter) in our Cargo.toml file. We also need to determine whether/how to use DNSSEC for the lookups after enabling this feature - it looks like we need to construct a ResolverOpts instance, then set its validate member to true, and then pass the instance into the resolver; then we just use the resolver's lookup method as usual.