feat(blob): Remove blob external storage mode

[lxy-9602]

Purpose

Linked issue: close #283

Remove the blob-external-storage-field and blob-external-storage-path functionality, following apache/paimon#8378.

This change:

• Removes the external-storage blob table options from public option definitions and CoreOptions.
• Removes external-storage blob schema validation.
• Deletes ExternalStorageBlobWriter and its write-path integration from AppendOnlyWriter.
• Simplifies BlobFileContext to only classify descriptor, view, inline, and regular blob-file fields.
• Simplifies blob writer plumbing by removing the now-unused WriteConsumer and blob path creator hooks.
• Keeps existing descriptor/blob-view/raw blob behavior unchanged.
• Keeps historical test data, but removes the deleted options from fixture schema files.

Tests

API and Format

This removes two public table options from the C++ API:

• blob-external-storage-field
• blob-external-storage-path

No storage format change is introduced. Existing BlobDescriptor data whose URI points outside the main table path can still be read as long as the referenced files exist.

Documentation

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