[cherry-pick] br: less split during restoring(tidb#27240)

[YuJuncen]

cherry-picking tidb#27240

===

Port of #1377

What problem does this PR solve?

Partially fix #1374

What is changed and how it works?

before：
--|-------t1 data-------|-----|---t2 data-------|
after: 
----------t1 data-------|---------t2 data-------|

Legends:
'|' the split point
'-' the key space

Also, downloadSST would find rewrite rules by file instead of region start key for now.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  The same case as Too many empty region after restoration of many small tables #1374, the region count reduced about 1/2.

Release note

BR would split less regions to reduce the number of empty region after restoration.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• kennytm

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[sre-bot]

No release note, Please follow https://github.com/pingcap/community/blob/master/contributors/release-note-checker.md

[YuJuncen]

BR:Backup:ErrBackupNoLeader]backup no leader" 🤔

/run-integration-test

[glorv]

/run-check_release_note

[kennytm]

/rebuild

[YuJuncen]

/merge

[ti-chi-bot]

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

Commit hash: f8cc37c