Get only livestate-supported plugin clients when flushing the livestate

[ffjlabo]

What this PR does:

This is the fix for pluginRegistry's logic.

• Fixed to check whether the plugin server supports LivestateService in advance, by using gRPC reflection.
• Get only livestate-supported plugin clients when flushing the livestate.

Why we need it:

Currently, the livestate reporter fails to report when there are some plugins that don't support the livestate feature. (e.g. wait stage).

Livestate reporter calls GetLivestate for each plugin when flushing an app.
But currently, it fails when one of them has an error, including an unimplemented status.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

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

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 54.94%. Comparing base (d73300d) to head (2f8aca5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/sdk/plugin.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #5940       +/-   ##
===========================================
+ Coverage   28.47%   54.94%   +26.46%     
===========================================
  Files         520       86      -434     
  Lines       56415     8538    -47877     
===========================================
- Hits        16063     4691    -11372     
+ Misses      39086     3525    -35561     
+ Partials     1266      322      -944     

Flag	Coverage Δ
.	?
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	49.20% <0.00%> (-0.03%)	⬇️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[ffjlabo]

I found the simple alternative way. So close this PR.