fix: fix write replace in grpcStruct

[harshachinta]

#2846 has refactored the result set class. However the writeReplace method was not updated. This PR fixes it.
Impact: This will break the import export functionality in Pantheon and dataflow templates.

The error is as follows

Caused by: java.lang.IllegalArgumentException: Unable to encode element '[string_value: "Singers5"
, string_value: "1"
, string_value: "CgFB"
, null_value: NULL_VALUE
]' with coder 'SerializableCoder(com.google.cloud.spanner.Struct)'.
    org.apache.beam.sdk.coders.Coder.getEncodedElementByteSize(Coder.java:300)
    org.apache.beam.sdk.coders.Coder.registerByteSizeObserver(Coder.java:291)
    org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$SampleByteSizeDistribution.tryUpdate(PCollectionConsumerRegistry.java:509)
    org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:336)
    org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:275)
Caused by: java.lang.ClassCastException: class com.google.protobuf.Value cannot be cast to class java.lang.String (com.google.protobuf.Value is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
    com.google.cloud.spanner.GrpcStruct.writeReplace(GrpcStruct.java:92)

Job link: https://pantheon.corp.google.com/dataflow/jobs/us-central1/2024-02-21_00_14_12-8497790410952556713;bottomTab=JOB_LOGS;logsSeverity=ERROR;graphView=0?project=span-cloud-testing&pageState=(%22dfTime%22:(%22l%22:%22dfJobMaxTime%22))&mods=logs_tg_staging