Enable FIPS entitlements based on org.bouncycastle.fips.approved_only. (elastic#124577)

When enabling FIPS `javax.net.ssl.trustStore` is not necessarily set.
This change adds FIPS entitlements based on
`org.bouncycastle.fips.approved_only=true`, which enforces usage of FIPS
approved functionality only.

Additionally, this PR grants read access to a custom trust store if
provided via `javax.net.ssl.trustStore`, otherwise read access to the
default JDK trust store is granted.

Relates to ES-11025.

Author: mosche

libs/entitlement/src/main/java/org/elasticsearch/entitlement/initialization/EntitlementInitialization.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.entitlement.initialization;
 
+import org.elasticsearch.core.Booleans;
 import org.elasticsearch.core.PathUtils;
 import org.elasticsearch.core.internal.provider.ProviderLocator;
 import org.elasticsearch.entitlement.bootstrap.EntitlementBootstrap;
@@ -241,16 +242,22 @@ private static PolicyManager createPolicyManager() {
             )
         );
 
-        Path trustStorePath = trustStorePath();
-        if (trustStorePath != null) {
+        // conditionally add FIPS entitlements if FIPS only functionality is enforced
+        if (Booleans.parseBoolean(System.getProperty("org.bouncycastle.fips.approved_only"), false)) {
+            // if custom trust store is set, grant read access to its location, otherwise use the default JDK trust store
+            String trustStore = System.getProperty("javax.net.ssl.trustStore");
+            Path trustStorePath = trustStore != null
+                ? Path.of(trustStore)
+                : Path.of(System.getProperty("java.home")).resolve("lib/security/jssecacerts");
+
             Collections.addAll(
                 serverScopes,
                 new Scope(
                     "org.bouncycastle.fips.tls",
                     List.of(
                         new FilesEntitlement(List.of(FileData.ofPath(trustStorePath, READ))),
-                        new OutboundNetworkEntitlement(),
-                        new ManageThreadsEntitlement()
+                        new ManageThreadsEntitlement(),
+                        new OutboundNetworkEntitlement()
                     )
                 ),
                 new Scope(
@@ -302,11 +309,6 @@ private static Path getUserHome() {
         return PathUtils.get(userHome);
     }
 
-    private static Path trustStorePath() {
-        String trustStore = System.getProperty("javax.net.ssl.trustStore");
-        return trustStore != null ? Path.of(trustStore) : null;
-    }
-
     private static Stream<InstrumentationService.InstrumentationInfo> fileSystemProviderChecks() throws ClassNotFoundException,
         NoSuchMethodException {
         var fileSystemProviderClass = FileSystems.getDefault().provider().getClass();