[Feature] Support columnar-extend storage layout for MAP columns

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/paimon-cpp/issues) and found nothing similar.

### Motivation

In time-series / IoT / observability workloads, a common pattern is storing free-schema fields in a `MAP` column (e.g. `metrics MAP`). The default MAP storage (two KV arrays) provides:

- No per-key columnar access
- No per-key statistics
- No predicate pushdown on individual keys

This makes queries like `SELECT ext_map['usage'] FROM metrics WHERE ext_map['usage'] > 30` scan the entire MAP column — extremely inefficient when only 1–3 keys out of thousands are needed per query.

The [PIP-43: Columnar Storage Optimization for MAP Type in Paimon](https://cwiki.apache.org/confluence/display/PAIMON/PIP-43%3A+Columnar+Storage+Optimization+for+MAP+Type+in+Paimon) proposes a new `shared-shredding` storage layout that stores MAP values in `K` reusable physical columns within a Struct, achieving near-full columnar access with per-key statistics and predicate pushdown — without changing the logical type (`MAP`).

### Solution

#### Physical Layout

Each `MAP` column configured with `fields..map.storage-layout = shared-shredding` is physically stored as:

```
STRUCT<
__field_mapping: FixedSizeList, -- per-row: which field_id each col holds
__col_0: T, __col_1: T, ..., __col_{K-1}: T, -- reusable typed columns
__overflow: MAP -- rare fallback for rows with > K fields
>
```

`fields..map.shared-shredding.max-columns` controls K_max, and `fields..map.shared-shredding.column-placement-policy` controls column placement.

File metadata (footer) stores: field name↔id dictionary, field_id→physical column set S, overflow set O, K, and max row width.

#### Write Path

1. **Schema conversion utilities** — Logical MAP → physical Struct schema rewriting; metadata serialization/deserialization; shared-shredding column detection via field metadata marker.

2. **`FormatWriter::AddMetadata`** — New virtual method (default no-op) for writing key-value metadata to file footer before `Finish()`. Parquet implementation calls `AddKeyValueMetadata`.

3. **Column allocator** — Per-row slot allocator that maps field IDs to up to `K` physical columns and sends the rest to overflow. Placement policy is configurable (`plain`, `sequential`, `lru`; default `plain`). Accumulates file-level statistics (S, O, max row width).

4. **Logical→physical batch converter** — Parses logical MAP, encodes field names to integer IDs (file-level dictionary), invokes allocator per row, assembles physical Struct array.

5. **Writer integration** — Extended DataFileWriter that performs conversion before writing + injects metadata on close. AppendOnlyWriter detects shared-shredding columns and routes accordingly. Cross-file K adaptation (P99 of recent max row widths, capped by K_max).

#### Read Path

1. **File metadata parsing** — Parse shared-shredding metadata from file footer (dictionary, S, O, K). New `GetFileKeyValueMetadata()` method on `FileBatchReader` with Parquet implementation.

2. **Predicate translation** — Translate logical predicates on MAP keys into conservative OR predicates over physical sub-columns. Requires extending `LeafPredicate` to support nested field paths and updating `PredicateConverter` to emit nested `FieldRef`.

3. **Read planning** — At `SetReadSchema` time: look up which physical columns to read (from S), decide whether `__overflow` is needed (from O), translate predicates, and pass the physical schema + physical predicate down to the inner `FileBatchReader` unchanged.

4. **Batch reconstruction** — After `NextBatch`: read `__field_mapping` per row to identify which column holds which field (fine-grained filter), gather values into logical `MAP`. Merge overflow when needed. Correctness relies on per-row `__field_mapping`, not on pushdown precision.

5. **Reader integration** — A wrapper reader (implements `FileBatchReader`) sits between the upper layer and the format-level reader. Per-file instance. Compatible with varying K across files. Orthogonal to `DataEvolutionFileReader` (schema evolution).

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!