Support setting customized envs for backup and restore jobs (#3833)

[ti-srebot]

cherry-pick #3833 to release-1.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb-operator repo:
git pr https://github.com/pingcap/tidb-operator/pull/3848

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

git push git@github.com:ti-srebot/tidb-operator.git pr/3848:release-1.1-d43d017b69a6

---

What problem does this PR solve?

Closes #3831

What is changed and how does it work?

Add Env field in BackupSpec, and add logic/ut cases for backup, restore and clean jobs.

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

Support setting customized envs for backup and restore jobs

[ti-srebot]

@dragonly you're already a collaborator in bot's repo.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (release-1.1@2b1056e). Click here to learn what that means.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             release-1.1    #3848   +/-   ##
==============================================
  Coverage               ?   51.86%           
==============================================
  Files                  ?      158           
  Lines                  ?    15339           
  Branches               ?        0           
==============================================
  Hits                   ?     7956           
  Misses                 ?     6620           
  Partials               ?      763           

Flag	Coverage Δ
unittest	51.86% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

[DanielZhangQD]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• DanielZhangQD

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by writing /lgtm in a comment.
Reviewer can cancel approval by writing /lgtm cancel in a comment.

[DanielZhangQD]

/merge

[ti-chi-bot]

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

Commit hash: 2fad0fa