[Feature] Export the pinned table's write Arrow schema through the C API #1328

Search before asking

• I searched the issues and pull requests for a matching schema-export API.

Motivation

A C/C++ caller must construct Arrow batches matching the Paimon table's write
schema. The C API can create a table from resolved schema JSON but does not
expose its corresponding Arrow schema. Reimplementing the type mapping in
each caller can diverge from the Rust writer's validation; querying a catalog
again can also select a different table schema.

Solution

Add an additive C function:

paimon_error *paimon_table_export_arrow_schema(
    const paimon_table *table, void *schema);

schema points to an initialized empty ArrowSchema. Reuse the core write
schema conversion and the Arrow C Data Interface, borrowing the table and
giving the output an independent release callback. Do not refresh catalog
metadata or change the core type rules. Reject null pointers and a live output
instead of overwriting it.

Anything else?

Cover type/order/nullability/field-ID mapping, a real write using the exported
schema, table/output lifetimes, release and reuse, and invalid pointers. This
proposal was prepared with AI assistance; it does not add another type mapping.

Willingness to contribute

• I'm willing to submit a PR!