feat: UNPIVOT supports AS

[forsaken628]

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Summary

Snowflake does not support this UNPIVOT AS syntax, but databricks does.

Example:

-- Create the unpivoted_monthly_sales table
CREATE TABLE unpivoted_monthly_sales(  empid INT,   jan INT,  feb INT,  mar INT,  apr INT);

-- Insert sales data
INSERT INTO unpivoted_monthly_sales VALUES
  (1, 10400,  8000, 11000, 18000),
  (2, 39500, 90700, 12000,  5300);

SELECT *
FROM unpivoted_monthly_sales
    UNPIVOT (amount
    FOR month IN (jan as 'Jan', feb AS 'F e b', mar 'MARCH', apr));
┌──────────────────────────────────────────────────────┐
│      empid      │       month      │      amount     │
│ Nullable(Int32) │ Nullable(String) │ Nullable(Int32) │
├─────────────────┼──────────────────┼─────────────────┤
│               1 │ Jan              │           10400 │
│               1 │ F e b            │            8000 │
│               1 │ MARCH            │           11000 │
│               1 │ apr              │           18000 │
│               2 │ Jan              │           39500 │
│               2 │ F e b            │           90700 │
│               2 │ MARCH            │           12000 │
│               2 │ apr              │            5300 │
└──────────────────────────────────────────────────────┘

• fixes: feat: UNPIVOT supports AS #17583

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[BohuTANG]

Doc has updated: databendlabs/databend-docs@63f8b37