feat: allow attempt direct path xds via env var

Author: HailongWen

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

@@ -354,6 +354,11 @@ public GapicSpannerRpc(final SpannerOptions options) {
                   options.isAttemptDirectPath()
                       && !Objects.equals(
                           options.getScopedCredentials(), NoCredentials.getInstance()));
+      String directPathXdsEnv = System.getenv("GOOGLE_SPANNER_ENABLE_DIRECT_ACCESS");
+      boolean isAttemptDirectPathXds = Boolean.parseBoolean(directPathXdsEnv);
+      if (isAttemptDirectPathXds) {
+        defaultChannelProviderBuilder.setAttemptDirectPathXds();
+      }
       if (options.isUseVirtualThreads()) {
         ExecutorService executor =
             tryCreateVirtualThreadPerTaskExecutor("spanner-virtual-grpc-executor");