feat: Add signatures to AgentCard for gRPC as per v0.3.0

Author: fjuma

spec-grpc/src/main/java/io/a2a/grpc/utils/ProtoUtils.java

@@ -18,6 +18,7 @@
 import io.a2a.spec.APIKeySecurityScheme;
 import io.a2a.spec.AgentCapabilities;
 import io.a2a.spec.AgentCard;
+import io.a2a.spec.AgentCardSignature;
 import io.a2a.spec.AgentExtension;
 import io.a2a.spec.AgentInterface;
 import io.a2a.spec.AgentProvider;
@@ -124,6 +125,9 @@ public static io.a2a.grpc.AgentCard agentCard(AgentCard agentCard) {
                 builder.addAllSkills(agentCard.skills().stream().map(ToProto::agentSkill).collect(Collectors.toList()));
             }
             builder.setSupportsAuthenticatedExtendedCard(agentCard.supportsAuthenticatedExtendedCard());
+            if (agentCard.signatures() != null) {
+                builder.addAllSignatures(agentCard.signatures().stream().map(ToProto::agentCardSignature).collect(Collectors.toList()));
+            }
             return builder.build();
         }
 
@@ -398,6 +402,16 @@ private static io.a2a.grpc.AgentSkill agentSkill(AgentSkill agentSkill) {
             return builder.build();
         }
 
+        private static io.a2a.grpc.AgentCardSignature agentCardSignature(AgentCardSignature agentCardSignature) {
+            io.a2a.grpc.AgentCardSignature.Builder builder = io.a2a.grpc.AgentCardSignature.newBuilder();
+            builder.setProtected(agentCardSignature.protectedHeader());
+            builder.setSignature(agentCardSignature.signature());
+            if (agentCardSignature.header() != null) {
+                builder.setHeader(struct(agentCardSignature.header()));
+            }
+            return builder.build();
+        }
+
         private static io.a2a.grpc.SecurityScheme securityScheme(SecurityScheme securityScheme) {
             io.a2a.grpc.SecurityScheme.Builder builder = io.a2a.grpc.SecurityScheme.newBuilder();
             if (securityScheme instanceof APIKeySecurityScheme) {