arrow

GH-37487: [C++][Parquet] Dataset: Implement sync `ParquetFileFormat::GetReader` (#37514)



### Rationale for this change

As https://github.com/apache/arrow/issues/37487 says. When thread cnt == 1, the thread might blocking in `ParquetFileFormat::GetReaderAsync`, that's because:

1. `ParquetFileFormat::CountRows` would call `EnsureCompleteMetadata` in `io_executor`
2. `EnsureCompleteMetadata` call `ParquetFileFormat::GetReader`, which dispatch real request to async mode
3. `async` is executed in `io_executor`.

1/3 in same fix-sized executor, causing deadlock.

### What changes are included in this PR?

Implement sync `ParquetFileFormat::GetReader`.

### Are these changes tested?

Currently not

### Are there any user-facing changes?

Bugfix

* Closes: #37487

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>