[CORE][VL] No Iceberg-specific switch to enable/disable Iceberg read and write offload

### Description

Gluten currently has **no Iceberg-specific configuration entry at all** — neither for read nor for
write. Searching the whole repository for `buildConf` / `buildStaticConf` / `registerConf` keys
containing `iceberg` returns nothing.

As a result, operators who need to turn Iceberg offload off (bad data, a validation gap, a
regression after an upgrade) have to reach for a switch that is far broader than Iceberg:

| Path | Only way to disable today | Problem |
|---|---|---|
| Read | `spark.gluten.sql.columnar.batchscan` | Generic DSv2 switch. Turning it off also falls back Paimon, Hudi and every other batch scan. |
| Write | `spark.gluten.sql.enable.enhancedFeatures` | A bundled flag (`VeloxConfig.scala`), documented as "iceberg native write **and other features**". Default `true`. |

#### Details

**Read.** `OffloadIcebergScan` (`gluten-iceberg/.../extension/OffloadIcebergScan.scala`) is gated
only by `Validators.FallbackByUserOptions`:

```scala
case p: BatchScanExec if !glutenConf.enableColumnarBatchScan => fail(p)
```

There is no way to express "fall back Iceberg scans but keep other DSv2 scans offloaded".

**Write.** The five `OffloadIcebergWrite` rules
(`backends-velox/src-iceberg/.../extension/OffloadIcebergWrite.scala`) are gated by
`VeloxBackend.supportAppendDataExec()` and friends, all of which return
`VeloxConfig.get.enableEnhancedFeatures()`. Two consequences:

1. The `enhancedFeatures` flag conflates "was the native lib built with enhanced features" with
"does the user want Iceberg native write". Today Iceberg write happens to be its only member, but
the doc string already says "and other features" — a second member removes the ability to target
Iceberg alone.
2. The build-time half of that gate is already gone: `isEnhancedFeaturesEnabled` in
`cpp/velox/jni/VeloxJniWrapper.cc` now unconditionally returns `true` (the
`#ifdef GLUTEN_ENABLE_ENHANCED_FEATURES` was dropped). Iceberg native write is therefore on by
default, with the bundled SQL conf as the only remaining gate. #12660 similarly notes "Iceberg is
not dependent on the enhanced tag anymore".
3. `supportAppendDataExec()` is also read once at **rule-injection time**
(`VeloxRuleApi.scala`, to decide whether to inject `RewriteCreateTableAsSelect`), so flipping
`enhancedFeatures` at runtime only affects the validator half — it is not a clean kill switch.

Iceberg write validation carries a long list of restrictions (Parquet only, no sort order, no
UUID/FIXED, no merge schema, no accept-any-schema, no float/double or nested partition columns,
no incompatible column names — see `IcebergWriteExec.doValidateInternal`). A write-path problem
means bad files rather than a slow query, so a switch that disables *only* Iceberg write, without
touching anything else, is worth having on its own.

#### Proposal

Add one switch per direction, both defaulting to `true` so behaviour is unchanged:

```
spark.gluten.sql.columnar.iceberg.enableNativeRead
spark.gluten.sql.columnar.backend.velox.iceberg.enableNativeWrite
```

The asymmetry in the key names is deliberate and follows where the code lives:

- `OffloadIcebergScan` is in `gluten-iceberg/` and is shared by **both** backends
(`VeloxIcebergComponent` and `CHIcebergComponent` both call it), so the read key must not carry a
`backend.velox` prefix.
- `OffloadIcebergWrite` is in `backends-velox/src-iceberg/` and is Velox-only. The write key mirrors
the existing Delta precedent, `spark.gluten.sql.columnar.backend.velox.delta.enableNativeWrite`
(`VeloxDeltaConfig`), which is checked with an early return in `OffloadDeltaCommand`.

Checks belong inside the offload rules (not at injection time), so the switches stay dynamic.
The write switch should be AND-ed with `enhancedFeatures` rather than replacing it.

One known gap to decide on: `AllVeloxConfiguration` only walks `VeloxConfig.allEntries`, so
component-level config objects are not picked up by the docs generator — which is why
`VeloxDeltaConfig`'s key is absent from `docs/velox-configuration.md` today. Either accept the gap
or extend the generator to cover component configs.

Related but distinct: #11703 (mapping Iceberg *table properties* to Velox write configs), #8953
(Iceberg support tracker).

This enhancement description was written with the assistance of AI tooling.

### Gluten version

main branch

Start with OffloadIcebergScan.scala and OffloadIcebergWrite.scala, then compare the VeloxDeltaConfig precedent and the gates in VeloxConfig.scala, VeloxBackend and VeloxRuleApi.scala. Check how configuration entries are registered and how AllVeloxConfiguration generates documentation. Done means separate dynamic read and write switches exist, default to true, and the write switch remains combined with enhancedFeatures.

Written by the indexing model from the issue text.