fix(Spanner): ensure tag is passed to BeginTransactionRequest

[bshaffer]

b/469259813

Ensure when a tag is set in the $options array for Database::runTransaction, that the tag is added to BeginTransactionRequest.

Example:

$options = ['tag' => $tagName];
$database->runTransaction(function (Transaction $transaction) use ($table) {
    // Create an insert mutation.
    $mutationData = [
        ['SingerId' => 35, 'FirstName' => 'Alice', 'LastName' => 'Smith'],
        ['SingerId' => 36, 'FirstName' => 'Bob', 'LastName' => 'Johnson'],
    ];

    foreach ($mutationData as $data) {
        $transaction->insert($table, $data);
        printf('Added insert mutation for SingerId: %s' . PHP_EOL, $data['SingerId']);
    }

    // Commit the transaction. 
    $commitTimestamp = $transaction->commit();
    printf('Committed transaction with tag: %s' . PHP_EOL, $commitTimestamp->formatAsString());
}, $options);