Add a stand-alone load mode to DM.

[OliverS929]

What problem does this PR solve?

Issue Number: close #9230

What is changed and how it works?

Primary work to add a stand-alone load mode to DM, alongside with dump mode. Load&Sync mode might not be needed anymore and could be depreciated in the future.

UT/IT added are under testing and will be pushed soon.

Continue the work from #11738 as its intended change in openapi conflicted with that of #11729.

Check List

Tests

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

Questions

Will it cause performance regression or break compatibility?

N/A

Do you need to update user documentation, design documentation or monitoring documentation?

Yes. Description of those standalone modes would need to be added to docs for openapi only. Support for dmctl will be delayed further.

Release note

Please refer to [Release Notes Language Style Guide](https://pingcap.github.io/tidb-dev-guide/contribute-to-tidb/release-notes-style-guide.html) to write a quality release note.

If you don't think this PR needs a release note then fill it with `None`.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign benmeadowcroft, d3hunter for approval, ensuring that each of them provides their approval before proceeding. For more information see the Code Review Process.

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

Needs approval from an approver in each of these files:

• dm/OWNERS
• dm/config/OWNERS
• engine/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 55.1517%. Comparing base (68c22f3) to head (7f75e36).

Additional details and impacted files

Components	Coverage Δ
cdc	59.5770% <ø> (∅)
dm	50.0288% <57.1428%> (∅)
engine	53.2618% <0.0000%> (+0.0395%)	⬆️

Flag	Coverage Δ
unit	55.1517% <50.0000%> (+1.9293%)	⬆️

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

@@               Coverage Diff                @@
##             master     #11749        +/-   ##
================================================
+ Coverage   53.2223%   55.1517%   +1.9293%     
================================================
  Files           213       1002       +789     
  Lines         17720     137401    +119681     
================================================
+ Hits           9431      75779     +66348     
- Misses         7686      56075     +48389     
- Partials        603       5547      +4944     

[OliverS929]

/test engine-integration-test

[OliverS929]

/test dm-integration-test

[OliverS929]

/retest

[River2000i]

how about adding the sync task after load task finish? For test the complete create DM task

[OliverS929]

Yes, we can definitely do that. I actually have a test written in E2E, which I'll submit in a separate PR. How about I also add that test to the OpenAPI test suite along with the E2E PR? Additionally, we could merge the dump test with the existing load test, as the load test above already covers the dump task.