[Feature](lance) Add namespace and table DDL support

[Gabriel39]

Parent

Tracked by #66340. The baseline Lance integration in #65730 is read-only: it supports namespace/table discovery and table description, but it does not expose Lance metadata mutation operations.

Current status

Doris currently maps the Lance Namespace SDK's read APIs as follows:

• SHOW DATABASES uses ListNamespaces.
• SHOW TABLES uses ListTables.
• DESC and table loading use DescribeTable.

LanceExternalCatalog does not currently provide ExternalMetadataOps, so namespace/table creation, schema changes, rename, and drop operations are not supported.

Scope for Doris 4.2

• Support CREATE DATABASE by creating a Lance Namespace.
• Support DROP DATABASE by dropping a Lance Namespace with well-defined non-empty and IF EXISTS behavior.
• Support CREATE TABLE by creating a Lance dataset/table and registering the catalog metadata.
• Support SHOW CREATE TABLE with an accurate Doris representation of the Lance schema and relevant table properties.
• Support ALTER TABLE schema evolution, including add, alter/backfill, rename, and drop column operations supported by Lance.
• Support RENAME TABLE.
• Support DROP TABLE, with explicit semantics for deleting the underlying dataset versus deregistering only the catalog entry.
• Refresh Doris metadata caches after successful mutations.
• Define behavior for filesystem and REST Namespace catalogs, authorization, concurrent updates, failures, and version consistency.

Row-level DML (INSERT, UPDATE, and DELETE) is not part of this issue.

Completion criteria

• A Lance implementation of Doris external metadata operations is integrated with FE DDL statements.
• Filesystem and REST Namespace behavior is covered where the corresponding Lance implementation supports the operation.
• FE unit tests and regression tests cover success, IF EXISTS/IF NOT EXISTS, conflicts, unsupported operations, and cache refresh.
• User documentation describes supported syntax, catalog-specific behavior, data-deletion semantics, and limitations.
• The implementation is merged into the Doris 4.2 branch and verified with representative Lance catalogs.