[Improvement] Unsupported Gravitino datatype: external(serial) when querying via Trino

### What would you like to be improved?

Description
Environment

Trino version: 478
Gravitino / Trino–Gravitino connector version: 1.2.0
Underlying catalog / connector: JDBC PostgreSQL through Gravitino metalake
Deployment: Docker
Problem

When I run a SQL query through Trino against a table registered in Gravitino, the query fails with:

Query 20260503_125210_00017_z5hsm failed: Unsupported gravitino datatype: external(serial)
The error appears to be raised while Trino resolves metadata / column types from Gravitino. It suggests that a column type is exposed to Trino as external(serial) (likely originating from PostgreSQL SERIAL / similar), but the Trino–Gravitino integration does not provide a mapping for this type.

Steps to reproduce

Register a PostgreSQL table that includes a SERIAL (or equivalent) column in Gravitino (describe how: JDBC catalog, metalake, etc.).
From Trino, run: (e.g. SELECT * FROM catalog.schema.table LIMIT 1 or the minimal query that fails).
Observe the failure with the message above.

### How should we improve?

Expected behavior
The query should succeed, or unsupported types should be documented with a clear workaround (e.g. mapped to INTEGER / BIGINT).

Actual behavior

The query fails immediately with Unsupported gravitino datatype: external(serial).

Start by reproducing the failure with Trino 478 against a PostgreSQL table registered through the JDBC PostgreSQL catalog in the Gravitino metalake, using a SERIAL column and a minimal SELECT. Trace how the Trino–Gravitino connector resolves the column metadata exposed as external(serial). Done means the query succeeds with an appropriate mapping, or the unsupported type has a documented workaround.

Written by the indexing model from the issue text.