refactor: split up candidate registration and keep alive

[CookiePieWw]

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Refer to a related PR or issue link (optional)

#5208

What's changed and what's your intention?

A refactor that has been discussed before. For etcd it's relatively complicated since the keep alive is operated through a keep alive stream returned by lease client. So the keep alive is still operated inside the candidate registration.

Not sure if it's the expected structure, so I just init a draft pr.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

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[codecov]

Codecov Report

Attention: Patch coverage is 47.61905% with 22 lines in your changes missing coverage. Please review.

Project coverage is 83.88%. Comparing base (c370b4b) to head (0e0ec53).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5320      +/-   ##
==========================================
- Coverage   84.14%   83.88%   -0.26%     
==========================================
  Files        1181     1181              
  Lines      221141   221161      +20     
==========================================
- Hits       186074   185528     -546     
- Misses      35067    35633     +566     

[CookiePieWw]

Candidate register and keep alive shoud be progressive, so it's simpler to put them into a function instead of split them up.