Support reading Paimon VECTOR type in paimon-rust

[JunRuiLee]

Search before asking

• I searched in the issues and found nothing similar.

Description

Motivation

Apache Paimon has a first-class VECTOR type (VECTOR<element, length>), and
tables written by Java/Python Paimon can contain VECTOR columns. paimon-rust
has no VectorType today, so it cannot read these tables.

In Paimon, VectorType is a general-purpose column type (it can be read,
projected, and written like any other column); it just happens to also be the
natural input to vector (ANN) search. So the goal here is full support for the
type, not only the read path.

Goal: give paimon-rust first-class support for Paimon VECTOR columns —
read, write, and vector (ANN) search — matching Java/Python Paimon.

Scope (incremental PRs)

Read foundation:

• PR 1 — Add native VectorType to the type system (variant, validation,
  JSON serde, Display).
• PR 2 — Arrow conversion (VectorType ↔ FixedSizeList) + read vector
  columns inlined in ordinary data files.
• PR 3 — Read dedicated .vector. files (parquet/vortex).

Search & write:

• PR 4 — Integrate VECTOR<FLOAT> with the existing vector (ANN) search
  path. Today lumina/vindex index build and VectorSearchBuilder only
  accept ARRAY<FLOAT>; extend them to also accept DataType::Vector, deriving
  the dimension from the column type (VectorType::length) instead of requiring
  an explicit dimension option. Matches the Java behavior where both
  ARRAY<FLOAT> and VECTOR<FLOAT> are valid vector-search columns.
• PR 5+ — Writing VECTOR columns (inline + dedicated .vector. files).

Willingness to contribute

• I'm willing to submit a PR!