lib: introduce Curl_peer

`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

scripts/schemetable.c

@@ -53,6 +53,11 @@ static const char *scheme[] = {
   "smbs",
   "smtp",
   "smtps",
+  "socks",
+  "socks4",
+  "socks4a",
+  "socks5",
+  "socks5h",
   "telnet",
   "tftp",
   "ws",