error: wrap io::Error in Arc for clone

[cpu]

ProtoErrorKind is Clone, but the Io variant holding io::Error runs into trouble with this: since the io::Error error can't be cloned we have to reconstruct it and this is a lossy proces resulting in a "simple" io::Error that only holds the error type from the parent it was cloned from. This loses important details like the underlying error source/message.

This branch changes ProtoErrorKind::Io to hold Arc<io::Error> instead (Note: this is a breaking change). This makes implementing Clone trivial - we clone the arc - and no error information is lost.

With this fix in place we get a much better error fidelity from failures like trying to perform DNS-over-HTTPS using Rustls but without a source of trust anchors (e.g. #2066). Previously the error was surfaced as:

failed to lookup HTTPS record type: ResolveError { kind: Proto(ProtoError { kind: Io(Kind(InvalidData)) }) }'

With this change in place, the error is now surfaced with much more detail:

failed to lookup HTTPS record type: ResolveError { kind: Proto(ProtoError { kind: Io(Custom { kind: InvalidData, error: InvalidCertificate(UnknownIssuer) }) }) }

Updates #2066

[djc]

Thanks, this is great!