Add `id()` as a public function on `Field`

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

Today, arrow-rs supports Parquet field-IDs via a heuristic based on a special key (`PARQUET:field_id`) in the field metadata. Instead, I would suggest bumping it to a method on Field itself.

**Describe the solution you'd like**

Add `field_id` as a function to `Field`.

**Describe alternatives you've considered**

Keep it as is today.

**Additional context**

Apache Iceberg heavily relies on field-IDs, and having to do `field.metadata().get(PARQUET_FIELD_ID_META_KEY)?` in the code itself feels odd. I think this should be part of Arrow itself.

Locate the Rust Field type and the existing PARQUET:field_id metadata lookup. Compare the requested public id() or field_id function with current callers, then verify that field IDs remain accessible without direct metadata access and run the relevant existing tests for Field and Parquet integration.

Written by the indexing model from the issue text.