[core][rest] support schema validation and infer for external paimon table

[XiaoHongbo-Hope]

[core][rest] Add schema validation and inference for REST catalog external tables

Purpose

Currently, REST catalog supported external tables but did not validate schema consistency between the filesystem and server-side schema. External tables always required explicit schema definition even when schema already existed in the filesystem.

This PR

Enhances REST catalog external table creation with schema inference and validation:

• Validates client-provided schema against filesystem schema before creating table metadata
• Enables creating external tables without explicit schema when schema exists
  in the location

Examples:
CREATE TABLE t2 (id INT, name STRING) USING paimon LOCATION 'path' (explicit schema)
CREATE TABLE t2 USING paimon LOCATION 'path' (schema inference from filesystem, newly supported)

other fix about external table

• Ignore NotImplementedException exception when show all tables by load sys.tables to avoid failure caused by external paimon table

Tests

PaimonExternalTableTest
RESTCatalogTest.testCreateExternalTableWithSchemaInference
RESTCatalogTest.testReadSystemTablesWithExternalTable

API and Format

Documentation

[liujiayi771]

Is my understanding correct? There will be support for two ways to create paimon external tables:

1. Explicitly specifying the schema, which must be consistent with the schema on the file system.
2. Having no prior knowledge of the schema and directly using the schema from the file system.

[XiaoHongbo-Hope]

Is my understanding correct? There will be support for two ways to create paimon external tables:

1. Explicitly specifying the schema, which must be consistent with the schema on the file system.
2. Having no prior knowledge of the schema and directly using the schema from the file system.

Totally correct. Should be similar with HiveCatalog

[JingsongLi]

+1