*: merging tidb-lightning (#665)

[ti-srebot]

cherry-pick #665 to release-5.0-rc
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/740

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/br.git pr/740:release-5.0-rc-8b9b626a22e4

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What problem does this PR solve?

Merging TiDB-Lightning repo into BR repo.

What is changed and how it works?

git subtree add -P pkg/lightning https://github.com/pingcap/br/pkg/lightning.git in subtree-lightning branch.

This PR targets to subtree-lightning branch for easier review, also it includes changes on import path and lightning tests (to pass BR CI).

Check List

Tests

• Unit test
• Integration test

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

Release Note

• No release note

[overvenus]

We are not going to merging tidb-lightning into release-5.0-rc.