Support parsing MySQL LOAD DATA with @ variable

[TherChenYang]

Fixes #30973.

Changes proposed in this pull request:

• Support parsing MySQL LOAD DATA with @ variable

SQL Case

LOAD DATA INFILE 'file.txt'
  INTO TABLE t1
  (column1, @var1)
  SET column2 = @var1/100;

LOAD DATA INFILE 'file.txt'
  INTO TABLE t1
  (column1, @dummy, column2, @dummy, column3);

EXECUTE STMT USING @skip, @numrows;

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[strongduanmu]

LGTM, merged.