Improve certificate manager

[rambohe-ch]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:
/kind bug
/kind documentation
/kind enhancement
/kind good-first-issue
/kind feature
/kind question
/kind design
/sig ai
/sig iot
/sig network
/sig storage

/kind enhancement

What this PR does / why we need it:

1. improve YurtCertficateManager Interface for provides all certificates management.
2. add func NewCertManagerFactoryWithFnAndStore(), so yurthub can use CertManagerFactory util funcs to generate certs.
3. add func PickHealthyServer() for MultipleBackendsHealthChecker interface, so rest config manager can get healthy server directly.
4. add two parameters: --discovery-token-ca-cert-hash and --discovery-token-unsafe-skip-ca-verification for yurthub, so end user can use these parameters and increase security for joining nodes.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

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In response to this:

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:
/kind bug
/kind documentation
/kind enhancement
/kind good-first-issue
/kind feature
/kind question
/kind design
/sig ai
/sig iot
/sig network
/sig storage

/kind enhancement

What this PR does / why we need it:

1. improve YurtCertficateManager Interface for provides all certificates management.
2. add func NewCertManagerFactoryWithFnAndStore(), so yurthub can use CertManagerFactory util funcs to generate certs.
3. add func PickHealthyServer() for MultipleBackendsHealthChecker interface, so rest config manager can get healthy server directly.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

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[openyurt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rambohe-ch

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• OWNERS [rambohe-ch]

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[codecov]

Codecov Report

Merging #1133 (f6a8fd7) into master (0aad742) will increase coverage by 1.40%.
The diff coverage is 59.82%.

@@            Coverage Diff             @@
##           master    #1133      +/-   ##
==========================================
+ Coverage   53.56%   54.97%   +1.40%     
==========================================
  Files          97       96       -1     
  Lines       12877    12582     -295     
==========================================
+ Hits         6898     6917      +19     
+ Misses       5398     5070     -328     
- Partials      581      595      +14     

Flag	Coverage Δ
unittests	54.97% <59.82%> (+1.40%)	⬆️

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

Impacted Files	Coverage Δ
pkg/util/certmanager/pki.go	19.46% <0.00%> (-0.36%)	⬇️
pkg/yurthub/healthchecker/fake_checker.go	0.00% <0.00%> (ø)
pkg/yurthub/healthchecker/health_checker.go	76.66% <0.00%> (-3.20%)	⬇️
pkg/yurthub/server/server.go	0.00% <0.00%> (ø)
pkg/yurthub/kubernetes/rest/config.go	64.00% <58.33%> (+2.46%)	⬆️
pkg/util/certmanager/factory/factory.go	40.62% <64.70%> (+13.70%)	⬆️
pkg/yurthub/certificate/token/token.go	64.81% <64.81%> (ø)
pkg/controller/certificates/csrapprover.go	20.12% <100.00%> (ø)
pkg/yurthub/server/certificate.go	61.53% <100.00%> (ø)
pkg/yurthub/util/util.go	72.98% <100.00%> (-7.18%)	⬇️
... and 4 more

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[rambohe-ch]

@Congrool PTAL

[rambohe-ch]

@Congrool comments are fixed, PTAL

[Congrool]

/lgtm