crypto/spiffe: adds a multi trust anchor selector

Adds a new trust anchors provider that returns different trust
anchors depending on the requested trust domain out of a pre-loaded
set.

crypto/spiffe/trustanchors/multi.go

@@ -0,0 +1,69 @@
+/*
+Copyright 2025 The Dapr Authors
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+    http://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package trustanchors
+
+import (
+	"context"
+	"errors"
+
+	"github.com/spiffe/go-spiffe/v2/bundle/x509bundle"
+	"github.com/spiffe/go-spiffe/v2/spiffeid"
+
+	"github.com/dapr/kit/concurrency"
+)
+
+type OptionsMulti struct {
+	TrustAnchors map[spiffeid.TrustDomain]Interface
+}
+
+// multi is a TrustAnchors implementation which uses multiple trust anchors
+// which are indexed by trust domain.
+type multi struct {
+	trustAnchors map[spiffeid.TrustDomain]Interface
+}
+
+func FromMulti(opts OptionsMulti) Interface {
+	return &multi{
+		trustAnchors: opts.TrustAnchors,
+	}
+}
+
+func (m *multi) Run(ctx context.Context) error {
+	r := concurrency.NewRunnerManager()
+	for _, ta := range m.trustAnchors {
+		if err := r.Add(ta.Run); err != nil {
+			return err
+		}
+	}
+
+	return r.Run(ctx)
+}
+
+func (m *multi) CurrentTrustAnchors(ctx context.Context) ([]byte, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (m *multi) GetX509BundleForTrustDomain(td spiffeid.TrustDomain) (*x509bundle.Bundle, error) {
+	for tad, ta := range m.trustAnchors {
+		if td.Compare(tad) == 0 {
+			return ta.GetX509BundleForTrustDomain(td)
+		}
+	}
+
+	return nil, errors.New("trust domain not found")
+}
+
+func (m *multi) Watch(ctx context.Context, ch chan<- []byte) {
+	return
+}