cf-dns: connection filter for DNS queries

New connection filter `cf-dns` that manages DNS queries. If hands
out addresses and HTTPS-RR records to anyone interested. Used by
HTTPS and IP happy eyeballing.

Information may become available *before* the libcurl "dns entry"
is complete, e.g. all queries have been answered. The cf-ip-happy
filter uses this information to start connection attempts as soon
as the first address is available.

The multi MSTATE_RESOLVING was removed. A new connection always
goes to MSTATE_CONNECTING. The connectdata bit `dns_resolved`
indicates when DNS information is complete. This is used for
error reporting and starting the progress meter.

Removed dns entries `data->state.dns[i]`, as the `cf-dns` filter
now keeps the reference now.

Many minor tweaks for making this work and pass address information
around safely.

Closes #21027

lib/asyn-ares.c

@@ -349,6 +349,19 @@ static timediff_t async_ares_poll_timeout(struct async_ares_ctx *ares,
     return 1000;
 }
 
+const struct Curl_addrinfo *
+Curl_async_get_ai(struct Curl_easy *data,
+                  struct Curl_resolv_async *async,
+                  int ai_family, unsigned int index)
+{
+  /* Not supported by our implementation yet. */
+  (void)data;
+  (void)async;
+  (void)ai_family;
+  (void)index;
+  return NULL;
+}
+
 /*
  * Curl_async_await()
  *