feat: add LanceNamespaceCatalog backed by Lance namespace API

[fightBoxing]

Summary

Closes #14 — Adds Lance namespace catalog support.

Design (addressing @jackye1995's review)

• No adapter layer — LanceNamespaceCatalog extends AbstractCatalog directly using LanceNamespace. No intermediate AbstractLanceNamespaceAdapter or LanceNamespaceAdapter wrappers.
• No name collision — LanceNamespaceCatalogFactory is a separate Flink CatalogFactory (type=lance-namespace), distinct from the existing LanceCatalogFactory (type=lance).
• Real backend test — Integration test uses DirectoryNamespace (no mocks).

Usage

CREATE CATALOG my_lance WITH (
    'type' = 'lance-namespace',
    'impl' = 'dir',
    'root' = '/tmp/lance-warehouse'
);
USE CATALOG my_lance;

Changes

• LanceNamespaceConfig — configuration for namespace integration
• LanceNamespaceCatalog — AbstractCatalog backed by LanceNamespace
• LanceNamespaceCatalogFactory — Flink SPI CatalogFactory
• Added lance-namespace-core:0.7.5, lance-namespace-apache-client:0.7.5
• Fixed arrow-memory-core version alignment (15.0.0 → 18.3.0)
• LanceNamespaceCatalogITCase — 3 integration tests (2 passing, 1 needs Arrow IPC schema fix)

Verification

[INFO] Lance Flink Connector .............................. SUCCESS
[INFO] Lance Flink Connector - Flink 1.18 ................. SUCCESS
[INFO] Lance Flink Connector - Flink 1.19 ................. SUCCESS
[INFO] Lance Flink Connector - Flink 1.20 ................. SUCCESS

Tests verified with real DirectoryNamespace backend:

• ✅ testListDatabases
• ✅ testCreateAndDropDatabase