core/dnsserver: support explicit registration for embedded hosts (#8525)

* core/dnsserver: test host-plugin embedding with forward

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: add a directive setter for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: support explicit server registration for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

---------

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

core/dnsserver/directives.go

@@ -0,0 +1,33 @@
+package dnsserver
+
+import (
+	"fmt"
+	"slices"
+)
+
+// SetDirectives replaces [Directives] with a copy of directives, in execution
+// order. Names must be nonempty and unique. On error, Directives is unchanged.
+// An empty or nil list disables all directives.
+//
+// This selects directives but does not import or register plugins. A host may
+// register its plugins after this call, before starting Caddy. Unregistered
+// directives used in a Corefile are rejected by Caddy during startup.
+//
+// The list is process-wide, not per instance. Call SetDirectives before starting
+// any servers, not while servers are running. The caller must serialize it with
+// all other access to Directives, including startup and reload. It is not a
+// runtime reconfiguration API.
+func SetDirectives(directives []string) error {
+	seen := make(map[string]struct{}, len(directives))
+	for i, name := range directives {
+		if name == "" {
+			return fmt.Errorf("empty directive name at index %d", i)
+		}
+		if _, ok := seen[name]; ok {
+			return fmt.Errorf("duplicate directive %q", name)
+		}
+		seen[name] = struct{}{}
+	}
+	Directives = slices.Clone(directives)
+	return nil
+}