Support extension type names in schema introspection #26866

Is your feature request related to a problem or challenge?

DESCRIBE, SHOW COLUMNS, and information_schema.columns report the Arrow storage type of a column, even when its field metadata identifies an extension type. For example, a UUID column is reported as FixedSizeBinary(16). This hides the logical type from users and tools inspecting the schema.

Downstream applications can register extension types and customize how their values are displayed, but there is no corresponding hook for their type names in schema introspection.

Describe the solution you'd like

Allow registered extension types to provide a display name, and use it consistently in:

• DESCRIBE output;
• information_schema.columns.data_type;
• SHOW COLUMNS, which reads information_schema.columns.

For example, a registered UUID type could display as UUID instead of FixedSizeBinary(16). The mechanism should also support downstream extension types, including parameterized names when their metadata requires them.

Resolve the name using the field's extension metadata and the query session's registry. Preserve existing storage-type display for ordinary fields and extensions without a registered name override. A naming hook on DFExtensionType could be cool.

Add SLT coverage for the introspection paths and fallback behavior using registered extension types.

Describe alternatives you've considered

Applications can rewrite introspection results themselves, but each application then has to handle the different schema-inspection paths. Showing the raw ARROW:extension:name would expose the type identifier, but would not allow a friendly or parameterized name.

Additional context

Currently, both plan_describe and information-schema column construction format field.data_type() directly. DFExtensionType has a value-formatter hook but no type-name hook.

Related work:

• Part of #12644 — Support Extension Types / User Defined Types in DataFusion.
• #20312 introduced the extension registry and value-formatting support.
• #21714 maps SQL UUID and JSON types to extension-tagged fields. This feature would expose those logical types during schema introspection, as well as types registered by downstream applications.
• #18277, closed without merging, explored displaying types with metadata in plan and scalar representations.