feat: enable ALTS hard bound token in DirectPath (#3904)

Author: rockspore

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpc.java

@@ -368,6 +368,10 @@ public GapicSpannerRpc(final SpannerOptions options) {
       boolean isAttemptDirectPathXds = isEnableDirectPathXdsEnv();
       if (isAttemptDirectPathXds) {
         defaultChannelProviderBuilder.setAttemptDirectPath(true);
+        // This will let the credentials try to fetch a hard-bound access token if the runtime
+        // environment supports it.
+        defaultChannelProviderBuilder.setAllowHardBoundTokenTypes(
+            Collections.singletonList(InstantiatingGrpcChannelProvider.HardBoundTokenTypes.ALTS));
         defaultChannelProviderBuilder.setAttemptDirectPathXds();
       }