fix(http): prevent SSRF in outbound webhook and HTTP endpoint requests

Webhook and HTTP message endpoint deliveries both flow through
Postal::HTTP, which parsed the user-supplied URL and connected to its
host with no address validation. An authenticated user could point a
webhook or endpoint at a private, loopback or link-local address (e.g.
127.0.0.1, 169.254.169.254 cloud metadata, RFC1918 hosts) and make the
server issue requests into its own internal network.

Add Postal::HTTP::AddressGuard, which resolves the destination host and
rejects private/loopback/link-local/reserved/multicast IPv4 and IPv6
addresses, then pins the connection to the validated address so it cannot
be redirected via a DNS-rebinding race. Administrators can permit specific
destinations via the new postal.allowed_request_destinations config option
(hostnames or IP/CIDR ranges).

Address selection only uses families this server can actually reach so we
do not pin to an IPv6 address on a host without IPv6 connectivity; IPv4 is
preferred for predictability. HTTPEndpoint now validates that its URL is a
well-formed HTTP(S) URL with a host.

lib/postal/http/blocked_destination_error.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Postal
+  module HTTP
+    # Raised when an outbound request would be sent to an address that is not
+    # permitted (a private, loopback, link-local or otherwise reserved address
+    # that has not been explicitly allowlisted). Used as an SSRF guard.
+    class BlockedDestinationError < StandardError
+    end
+  end
+end