feat(table): build a catalog-free Table from a portable TableDescriptor

[JunRuiLee]

Depends on apache/paimon#8819 (the Java producer side of this contract). Draft until that PR's TableDescriptor JSON is finalized — the golden fixture here is byte-identical to the one produced there, so it must be updated in lockstep if the Java wire form changes.

Purpose

No dedicated issue yet — this is the Rust reader half of a cross-language contract; the Java producer half is apache/paimon#8819.

Add a portable, versioned TableDescriptor and a catalog-free way to build a read-only Table from it, so a non-Java engine (e.g. Doris BE via the C FFI) can rebuild a table for scan/read without a catalog lookup. Today the only way to obtain a Table is Catalog::get_table, which re-resolves the table (risking schema/snapshot drift vs. what the planner used) and ties the reader to a filesystem-catalog layout.

The descriptor carries only what a reader needs — path + the full TableSchema + optional options/database/name/branch — as language-neutral JSON. It is byte-compatible with the Java TableDescriptorSerializer output (apache/paimon#8819).

Brief change log

• crates/paimon/src/table/descriptor.rs: TableDescriptor (serde DTO) with from_json (rejects unknown version and non-main branch; wraps serde errors as DataInvalid), and Table::from_descriptor_json (top-level path wins over schema.options["path"]; identifier requires both database+name or neither; plain FileIO; rest_env = None; descriptor options are not merged into table semantics in v1).
• bindings/c/src/table.rs: C FFI paimon_table_from_descriptor(json, options, len), mirroring the existing paimon_catalog_get_table handle/free contract, with an ABI signature guard; freed via paimon_table_free.
• Historical per-file schemas are still read from the immutable on-disk schema/schema-N by SchemaManager; the descriptor pins the planned/current schema only.

Tests

• table::descriptor unit tests (13): parse valid v1; reject unknown version / non-main / empty branch; missing-field & malformed JSON → DataInvalid; build table + path-option injection; identifier synthesis and partial-identifier rejection; descriptor options do not affect table semantics; and parses_java_produced_golden_v1 — parses the byte-identical Java-produced golden and asserts the ARRAY field reconstructs to DataType::Array.
• paimon-c FFI tests (3): build a table from a descriptor; reject unknown version; reject null JSON.

API and Format

• New public API: paimon::table::TableDescriptor + Table::from_descriptor_json, and C FFI paimon_table_from_descriptor (additive; existing symbols unchanged).
• New JSON descriptor format, versioned (version = 1). No change to any existing on-disk or wire format.

Documentation

None yet (new, evolving cross-language contract); docs can follow once it stabilizes together with apache/paimon#8819.