DNS proxy optimization for entries in DNS cache

[kdorosh]

we can return CNAMEs for hosts we have in our cache to short-path DNS resolution across /etc/resolv.conf resolvers

for context, see #599 (comment)

this is a potential nice optimization we can bring to ztunnel DNS impl in due time. the implementation would be similar to but not exactly match #599, as this PR was returning the CNAME on cache miss rather than cache hit

[daixiang0]

Fixed by #603