[C++][Java] When i use DatasetFileWriter::write method write a file, I can't specify a file name

### Describe the usage question you have. Please include as many useful details as possible.

I use this static method to write a parquet file into fs, I give empty partition array, so it will be in a file; and i give the `baseNameTemplate` arg is "test.parquet" to specify the filename, but it leads to an error: `basename_template did not contain '{i}'`.
```java
public static void write(BufferAllocator allocator, ArrowReader reader, FileFormat format, String uri,
String[] partitionColumns, int maxPartitions, String baseNameTemplate) {
try (final ArrowArrayStream stream = ArrowArrayStream.allocateNew(allocator)) {
Data.exportArrayStream(allocator, reader, stream);
JniWrapper.get().writeFromScannerToFile(stream.memoryAddress(),
format.id(), uri, partitionColumns, maxPartitions, baseNameTemplate);
}
}
```
read the cpp jni code and cpp dataset's code, the FileSystemDatasetWriteOptions::basename_template seems not supported to specify the file name without `{i}`.
```cpp
/// \brief Options for writing a dataset.
struct ARROW_DS_EXPORT FileSystemDatasetWriteOptions {
.
.
.
/// Template string used to generate fragment basenames.
/// {i} will be replaced by an auto incremented integer.
std::string basename_template;
.
.
.
}
```
If there is any method to specify the filename when none partition?

### Component(s)

Java

Inizia da Java DatasetFileWriter::write e dalla chiamata JNI a writeFromScannerToFile, quindi esamina C++ FileSystemDatasetWriteOptions::basename_template. Determina se una scrittura non partizionata può accettare un nome file esatto senza {i}; il lavoro è completato quando il comportamento supportato è stato stabilito e il percorso Java/C++ gestisce questo caso o lo documenta chiaramente.

Scritto dal modello di indicizzazione a partire dal testo della issue.