client/http: implement the marshaler interfaces for Rule/RuleOp

[JmPotato]

What problem does this PR solve?

Issue Number: ref #7300.

What is changed and how does it work?

Implement the marshaler interfaces for `Rule` and `RuleOP` to make sure we could set/get the correct start/end key.

Ref https://github.com/pingcap/tidb/blob/46d4231c8b0ade353b98572e7c2a015bddf940f4/pkg/ddl/placement/rule.go#L76-L91.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• HuSharp
• rleungx

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 submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[JmPotato]

/test build

[codecov]

Codecov Report

Merging #7462 (259b48d) into master (beedacb) will decrease coverage by 0.06%.
The diff coverage is 61.53%.

❗ Current head 259b48d differs from pull request most recent head a98d738. Consider uploading reports for the commit a98d738 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7462      +/-   ##
==========================================
- Coverage   74.31%   74.25%   -0.06%     
==========================================
  Files         451      452       +1     
  Lines       49757    49899     +142     
==========================================
+ Hits        36977    37054      +77     
- Misses       9456     9506      +50     
- Partials     3324     3339      +15     

Flag	Coverage Δ
unittests	74.25% <61.53%> (-0.06%)	⬇️

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

[HuSharp]

LGTM!

[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

If you have any questions about the PR merge process, please refer to pr process.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[ti-chi-bot]

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

Commit hash: a98d738