feat: LDContext is java.io.Serializable

lib/shared/common/src/main/java/com/launchdarkly/sdk/ArrayBuilder.java

@@ -1,5 +1,6 @@
 package com.launchdarkly.sdk;
 
+import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -8,7 +9,7 @@
  * <p>
  * Builder methods are not thread-safe.
  */
-public final class ArrayBuilder {
+public final class ArrayBuilder implements Serializable {
   private List<LDValue> builder = new ArrayList<>();
   private volatile boolean copyOnWrite = false;
 

lib/shared/common/src/main/java/com/launchdarkly/sdk/AttributeMap.java

@@ -1,9 +1,10 @@
 package com.launchdarkly.sdk;
 
+import java.io.Serializable;
 import java.util.HashMap;
 import java.util.Map;
 
-final class AttributeMap {
+final class AttributeMap implements Serializable {
   private final AttributeMap parent;
   private final Map<String, LDValue> map;
 

lib/shared/common/src/main/java/com/launchdarkly/sdk/json/JsonSerializable.java

@@ -1,10 +1,12 @@
 package com.launchdarkly.sdk.json;
 
+import java.io.Serializable;
+
 /**
  * Marker interface for SDK classes that have a custom JSON serialization.
  * 
  * @see JsonSerialization
  * @see LDGson
  */
-public interface JsonSerializable { 
+public interface JsonSerializable extends Serializable {
 }