`struct Curl_peer` keeps information about a communication endpoint
together. It will replace `conn->host` and `conn->conn_to_host` and
proxyinfo host. It will also become part of `struct ssl_peer`.
It has a reference counter, so an instance can be shared between
connections and filters.
Elminiates `conn->host` and `conn->connect_to_host`, used in the
proxyinfo structures. Passed to DNS resolution and socks filters, etc.
Pass peer to http proxy and socks tunnel filters. Use peer in dns filter
and resolving. Make `Curl_peer` a member in the `struct ssl_peer`.
Add `docs/internals/PEERS.md` for documentation.
Closes#21472
