refactor: server manager

[bzp2010]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?

Refactoring the management code of Manager API to help to initialize, start and stop. Reduce redundant code and improve readability.

I created a new server module in internal/core.
It will help run according to the process of NewServer()->server.init()->Start()->Stop().
It covers the functions of initializing etcd client, initializing store, initializing route and starting HTTP server.

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

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[codecov-commenter]

Codecov Report

Merging #1956 (eeb8bbc) into master (4612d02) will decrease coverage by 3.34%.
The diff coverage is 50.51%.

@@            Coverage Diff             @@
##           master    #1956      +/-   ##
==========================================
- Coverage   66.89%   63.54%   -3.35%     
==========================================
  Files         179       60     -119     
  Lines        6551     3421    -3130     
  Branches      753        0     -753     
==========================================
- Hits         4382     2174    -2208     
+ Misses       1899      967     -932     
- Partials      270      280      +10     

Flag	Coverage Δ
backend-e2e-test	44.72% <50.51%> (+0.09%)	⬆️
backend-e2e-test-ginkgo	?
backend-unit-test	52.26% <0.00%> (-0.06%)	⬇️
frontend-e2e-test	?

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

Impacted Files	Coverage Δ
api/cmd/version.go	50.00% <0.00%> (-16.67%)	⬇️
api/internal/core/server/store.go	14.28% <14.28%> (ø)
api/internal/core/server/server.go	49.05% <49.05%> (ø)
api/internal/core/server/http.go	54.54% <54.54%> (ø)
api/cmd/root.go	78.04% <63.63%> (+18.45%)	⬆️
api/internal/utils/version.go	100.00% <100.00%> (ø)
api/internal/core/migrate/conflict.go	0.00% <0.00%> (-76.93%)	⬇️
api/internal/core/migrate/migrate.go	0.00% <0.00%> (-75.00%)	⬇️
api/internal/core/migrate/dataset.go	0.00% <0.00%> (-52.73%)	⬇️
api/internal/handler/migrate/migrate.go	5.55% <0.00%> (-51.86%)	⬇️
... and 140 more

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[bzp2010]

Hi, @tokers
If you have free time, can you review it? I have fixed the previous problems.
Thanks!