feat: add Arch Linux package (PKGBUILD, systemd service, sysusers)

- PKGBUILD for building the package
- Systemd service with CAP_NET_BIND_SERVICE for port 53
- sysusers.d config to create doh-forwarder system user
- Default config at /etc/doh-forwarder/config.toml

pkg/arch/doh-forwarder.conf

@@ -0,0 +1,17 @@
+[server]
+listen = "0.0.0.0:8800"
+
+[dns]
+listen = "0.0.0.0:53"
+
+[upstream]
+resolvers = [
+    { name = "cloudflare", url = "https://cloudflare-dns.com/dns-query" },
+    { name = "google", url = "https://dns.google/dns-query" },
+]
+strategy = "round-robin"
+
+[cache]
+enabled = true
+max_entries = 10000
+max_ttl_secs = 3600