feat: allow attempt direct path xds via env var (googleapis#2950)

To enable Direct Access, [both `setAttemptDirectPath` and `setAttemptDirectPathXds` should be called](https://togithub.com/googleapis/sdk-platform-java/blob/4b44a7851dc1d4fd2ac21a54df6c24db5625223c/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java#L373-L386) for gax to append the correct google-c2p scheme.

This PR adds a env var `GOOGLE_SPANNER_ENABLE_DIRECT_ACCESS` to control the enable/disable of Direct Access. When it is true, it calls `setAttemptDirectPathXds` which effectively turns on Direct Access (as `options.isAttemptDirectPath` is by default true and we don't need to call `setAttemptDirectPath` again).

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");