spanner: add support for struct in client executor - extractRowArrayValue

The extractRowArrayValue in executor converts a single column's array value at given index `i` to executor value.
However, the current methid lacks conversion when the type is TYPECODE_STRUCT.
https://github.com/googleapis/google-cloud-go/blob/f087721b7b20ad28ded1d0a84756a8bbaa2bb95a/spanner/test/cloudexecutor/executor/internal/utility/spanner_to_executor_value_converter.go#L163

b/382335717
The systests failed with the below error
```
ABORTED: RandomBatchTransactions (workid: Writes-auto-1733354544808703-184) failed: ABORTED: INVALID_ARGUMENT: Execution failed for batch partition, Failed action: database_path: "projects/spanner-cloud-systest/instances/cloud-systest0/databases/read_write_test" execute_partition { partition { cloud { cloud_partition: "\376\026\235\n\000\376\026\230AAeMGCkgu1tMiWfkcmWr9vrj__ReDkWrgi_78TXrybZ2wxPpd-BoeThAJWw-QdactF0bWA8IS4aUqaLq3yq4cF_w-acMGAH5-3GXBlDkQ9ath3NFGeRtZj28x1dBb96I3azzxYYp1z33WfzihkF_4WaYFW2VX58LQymm2953Huz3N_cuFH9m7KVCd2LH1YuBeBojhaJqn-rL9Nkb06IAtaJXe3HEbkW8tvIjXJbw3KVPvyvljSA5Eilizt7q_HUH-wjdPNyUqgVC3YSOEHyjpGMqGKm5ZUE5BDR6E94YDS1haLhhPE_ZB0eM4oelQ46fwhJ...[Action Truncated]: spanner: code = "InvalidArgument", desc = "extractRowArrayValue: unable to extract value: type STRUCT not supported"
Transaction Writes-auto-1733354544808703-184/137534 @1733355445173947 (2024/12/04-15:37:25.173947-08:00) schema_generation=3 dma_ctx=no concurrency=batch=1733355445173947:
Read log scopes: [Folders(5,0q*)..Folders(5,0q*)]
Query:
SET sql_version 1;
BIND p0 false;
BIND p1 NUMERIC "-24418225444244624169241243163.783041211";
SELECT
projectscan_6.a_2 AS expr0_7,
COALESCE(ARRAY< STRUCT< int64_f INT64 > >[], projectscan_6.a_5) AS expr1_10
FROM
(
SELECT
(CAST(@p1 AS STRING) IN (
SELECT
SAFE_CAST(JSON_VALUE('{"array":[null,3.974676704235172e+307],"bool":true,"null":null,"number_float":1.1206932170878527e+307,"number_integer":-7917485124678447281,"number_unsigned":16839281082195424947,"object":{"array":[-9006197925947932233],"string":"RQ3eN2PEG:RxD6NSD6XXghNyUNbhK5rQAnS"},"string":"l3HUGxIykZQGt7snzfsGVg2DQPDO7i4nssqwp4nNQGojS0GTVg"}',
"$.object.string") AS STRING) AS a_4
FROM
(
SELECT
1
) AS singlerowscan_3
WHERE
@p0
)) AS a_2,
ARRAY< STRUCT< int64_f INT64 > >[STRUCT< int64_f INT64 > (-6524593091844119782), STRUCT< int64_f INT64 >
(-101546339316740543), STRUCT< int64_f INT64 > (1496238184691556105), STRUCT< int64_f INT64 > (-3132155766874334742)] AS a_5
FROM
(
SELECT
1
) AS singlerowscan_1
WHERE
true
) AS projectscan_6;
To debug this, you may want to replay the batch transaction with the following command, adding a watch table and keys for the rows that differ:
RUN: spanner/automatic_build/systest/fetch_and_run_driver.sh systest_1204_14_702859027 read_write --systest_id=cloudclientquickgo-ra-1204-14 --spanner_systest_label=cloudclientquickgo --spanner_systest_cell=ra --systest_port=0 --client_mdb_group=spanner-test --systest_extra_command_arg=--spanner_test_profile=systest:group=system,binary=read_write_worker,script=read_write,label=cloudclientquickgo --systest_extra_command_arg=--spandex_enable_systestable_query_features --systest_extra_command_arg=--spanner_systest_prob_partition_hint=0.8 --cloud replaybatchtransaction projects/spanner-cloud-systest/instances/cloud-systest0/databases/read_write_test readonly 1733355445173947 3 Writes-auto-1733354544808703-184 137534 [ ]
=== Source Location Trace: ===
spanner/systest/handlers/read_write.cc:3152
[type.googleapis.com/util.ErrorSpacePayload='storage_systest::SYSTEST_FAILED']

```