Config-level forwarding rules were parsed with the UDP-only
`parse_upstream_addr` helper, silently rejecting the DoT/DoH schemes
that the rest of the forwarding pipeline already supports.
Widen `ForwardingRule.upstream` from `SocketAddr` to `Upstream` so
config rules reuse the same parser as `[upstream].address` and
`fallback`. Demote `parse_upstream_addr` to `pub(crate)` to prevent
the same mistake recurring.
Closes#100.
