`ParquetMetaDataWriter` silently omits page indexes from custom `PageIndexProvider`s

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

- Follow on to https://github.com/apache/arrow-rs/pull/10842 from @etseidl

We added the [`PageIndexProvider`](https://github.com/apache/arrow-rs/blob/79dbf5a3524cb8077d58eaec21c17f97de4ba4ca/parquet/src/file/metadata/page_index.rs#L139) trait so users can supply custom page index representations. However, when a `ParquetMetaData` holds a custom `PageIndexProvider` implementation, [`ParquetMetaDataWriter::finish`](https://github.com/apache/arrow-rs/blob/79dbf5a3524cb8077d58eaec21c17f97de4ba4ca/parquet/src/file/metadata/writer.rs#L444) **silently omits the page indexes** (`ColumnIndex` / `OffsetIndex`) from the output.

This means a user who:
1. Reads parquet metadata using a custom `PageIndexProvider`, and
2. Writes that metadata back out with `ParquetMetaDataWriter`

gets a file with **no page indexes** — with no error or warning. Readers of that file can no longer do page-level pruning, silently degrading query performance.

The cause is that the writer serializes the indexes by downcasting to the concrete built-in `PageIndex` type, which fails for custom providers:
https://github.com/apache/arrow-rs/blob/79dbf5a3524cb8077d58eaec21c17f97de4ba4ca/parquet/src/file/metadata/writer.rs#L464-L474

```rust
// Downcast to PageIndex to access raw index structures for serialization
// TODO: rework the encoder to work with the PageIndexProvider API to
// remove the need for this cast. Because the page index is a dyn trait,
// it can't be cloned. An implementation would have to create a new
// `PageIndex` from the current `PageIndexProvider` and pass that to
// the encoder.
if let Some(page_index_arc) = self.metadata.page_index.as_ref()
&& let Some(page_index) = page_index_arc
.as_any()
.downcast_ref::()
```

**Describe the solution you'd like**

Rework the metadata thrift encoder to work with the `PageIndexProvider` API directly, so page indexes are written regardless of the concrete provider implementation.

**Describe alternatives you've considered**

- Materialize a built-in `PageIndex` from any custom provider before serialization (simpler, but requires an extra copy of the index data).
- Keep the limitation but make it visible: document it on `ParquetMetaDataWriter` and/or return an error instead of silently omitting the indexes.

**Additional context**

- Original review discussion: https://github.com/apache/arrow-rs/pull/10842#discussion_r3913829496