[Plugin SDK] Change ApplicationLiveState.HealthStatus to private method

[Warashi]

What this PR does:

Implement application health status calculation in the SDK.

Why we need it:

To provide consistent application health state across all plugins.

Which issue(s) this PR fixes:

Part of #5530

Does this PR introduce a user-facing change?: No

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 27.63%. Comparing base (2047e9a) to head (0dbaa22).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../app/pipedv1/plugin/kubernetes/livestate/plugin.go	0.00%	1 Missing ⚠️
...plugin/kubernetes_multicluster/livestate/plugin.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5829      +/-   ##
==========================================
+ Coverage   27.62%   27.63%   +0.01%     
==========================================
  Files         508      508              
  Lines       54116    54117       +1     
==========================================
+ Hits        14948    14954       +6     
+ Misses      38031    38026       -5     
  Partials     1137     1137              

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[ffjlabo]

Thank you! LGTM