lightning: support data files with bom header (#40813)

[ti-chi-bot]

This is an automated cherry-pick of #40813

What problem does this PR solve?

Issue Number: close #40744

Problem Summary:

What is changed and how it works?

When reading the CSV / SQL data file's first block, trim the BOM header, and update the processed position.
Originally I planned to use a wrapped Reader to handle that. However, this will cause the processed position in parser not updated, thus resulting in abnormalities.
For example, when the file has a BOM header, after processing the first line data with 10 bytes, the parser position should be set to 10 + 3 = 13 , not 10. If set to 10, after processing all the file data, the final position is not equal to the chunk end offset, which is calculated by the file size. Then this termination if clause won't be hit:

tidb/br/pkg/lightning/restore/restore.go

Lines 2722 to 2725 in 4686338

	if kvSize >= minDeliverBytes || len(kvPacket) >= maxKvPairsCnt || newOffset == cr.chunk.Chunk.EndOffset {
	canDeliver = true
	kvSize = 0
	}

. That will cause the parser re-scan some file data again.
So the data manipulation is handled when reading data blocks. In this way we can also update the processed positions in the block parser.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• dsdashun
• gozssky

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[dsdashun]

/run-build

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/run-build

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/retest-required

[dsdashun]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: de917c3

[dsdashun]

/retest-required

[dsdashun]

/retest-required

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/retest-required

[dsdashun]

/retest-required

[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev	de917c3	link	true	/test check-dev
idc-jenkins-ci-tidb/mysql-test	de917c3	link	unknown	/test mysql-test

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