feat: allow non default service account in DirectPath (#2312)

Copying over from: https://togithub.com/googleapis/java-bigtable/pull/2012

Bigtable already sets the flag to allow non-default SA in DirectPath on the service side, we also need to set the option on the client side to make this feature work.
Related Spanner PR: https://togithub.com/googleapis/java-spanner/pull/2635.

Author: meeral-k

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/EnhancedBigtableStubSettings.java

@@ -370,7 +370,11 @@ public static InstantiatingGrpcChannelProvider.Builder defaultGrpcTransportProvi
     if (isDirectpathEnabled) {
       // Attempts direct access to CBT service over gRPC to improve throughput,
       // whether the attempt is allowed is totally controlled by service owner.
-      grpcTransportProviderBuilder.setAttemptDirectPathXds().setAttemptDirectPath(true);
+      grpcTransportProviderBuilder
+          .setAttemptDirectPathXds()
+          .setAttemptDirectPath(true)
+          // Allow using non-default service account in DirectPath.
+          .setAllowNonDefaultServiceAccount(true);
     }
     return grpcTransportProviderBuilder
         .setChannelPoolSettings(