dumpling: Parallel processing implementation for string-type

[takaidohigasi]

Enhancement

Currently, parallelization supports only numeric types for dumpling. so support string value.

related implementation

• pickupPossibleField
  https://github.com/pingcap/tidb/blob/master/dumpling/export/sql.go#L1286-L1299
• estimateCount
  https://github.com/pingcap/tidb/blob/master/dumpling/export/sql.go#L1301
  https://github.com/pingcap/tidb/blob/master/dumpling/export/sql.go#L1306
• detectEstimateRows
  https://github.com/pingcap/tidb/blob/master/dumpling/export/sql.go#L1343

Pull Request

#62172

[takaidohigasi]

/assign