Make the available ScriptContexts accessible from ScriptService (#70465)

Adds a method to the ScriptService to return the available Map of script context names to 
ScriptContexts. Also makes the ScriptEngines and ScriptContexts unmodifiable Maps on assignment 
since these should never be modified after the ScriptService is created.

server/src/main/java/org/elasticsearch/script/ScriptService.java

@@ -105,8 +105,8 @@ public class ScriptService implements Closeable, ClusterStateApplier {
     final AtomicReference<CacheHolder> cacheHolder = new AtomicReference<>();
 
     public ScriptService(Settings settings, Map<String, ScriptEngine> engines, Map<String, ScriptContext<?>> contexts) {
-        this.engines = Objects.requireNonNull(engines);
-        this.contexts = Objects.requireNonNull(contexts);
+        this.engines = Collections.unmodifiableMap(Objects.requireNonNull(engines));
+        this.contexts = Collections.unmodifiableMap(Objects.requireNonNull(contexts));
 
         if (Strings.hasLength(settings.get(DISABLE_DYNAMIC_SCRIPTING_SETTING))) {
             throw new IllegalArgumentException(DISABLE_DYNAMIC_SCRIPTING_SETTING + " is not a supported setting, replace with " +
@@ -246,6 +246,13 @@ public class ScriptService implements Closeable, ClusterStateApplier {
         IOUtils.close(engines.values());
     }
 
+    /**
+     * @return an unmodifiable {@link Map} of available script context names to {@link ScriptContext}s
+     */
+    public Map<String, ScriptContext<?>> getScriptContexts() {
+        return contexts;
+    }
+
     private ScriptEngine getEngine(String lang) {
         ScriptEngine scriptEngine = engines.get(lang);
         if (scriptEngine == null) {
@@ -270,7 +277,7 @@ public class ScriptService implements Closeable, ClusterStateApplier {
         maxSizeInBytes = newMaxSizeInBytes;
     }
 
-    /*
+    /**
      * Compiles a script using the given context.
      *
      * @return a compiled script which may be used to construct instances of a script for the given context