lightning should cache the result of columnAPI.Cols() to improve performance

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

capture a CPU profile when lightning encodes KV

tidb/br/pkg/lightning/backend/kv/sql2kv.go

Lines 491 to 497 in ff2feb6

	func (kvcodec *tableKVEncoder) getActualDatum(rowID int64, colIndex int, inputDatum *types.Datum) (types.Datum, error) {
	var (
	value types.Datum
	err error
	)
	cols := kvcodec.tbl.Cols()

some implementations of columnAPI.Cols() just return the slice, like before #50062 or after #53798. The other will filter and clone the slice.

2. What did you expect to see? (Required)

stable performance for different versions of lightning

3. What did you see instead (Required)

the duration doubles

4. What is your TiDB version? (Required)

at least v6.5.10