Made RequiredAttributeAccessor constructor public

Related to #422,606

Author: olegz

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/cloudevent/RequiredAttributeAccessor.java

@@ -34,12 +34,12 @@ public class RequiredAttributeAccessor extends CloudEventAttributesHelper {
 	 */
 	private static final long serialVersionUID = 859410409447601477L;
 
-	RequiredAttributeAccessor(Map<String, Object> headers, String prefixToUse) {
+	public RequiredAttributeAccessor(Map<String, Object> headers, String prefixToUse) {
 		super(headers);
 		this.prefixToUse = prefixToUse;
 	}
 
-	RequiredAttributeAccessor(Map<String, Object> headers) {
+	public RequiredAttributeAccessor(Map<String, Object> headers) {
 		this(headers, null);
 	}