Add area/plugin-sdk labeler rule

[t-kikuc]

What this PR does:

Automatically put the label to pkg/plugin/sdk/**/* files (recursive).

Why we need it:

To clarify which PRs are related to SDK updates.

Which issue(s) this PR 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

All modified and coverable lines are covered by tests ✅

Project coverage is 28.47%. Comparing base (38b047b) to head (37c219e).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5924   +/-   ##
=======================================
  Coverage   28.46%   28.47%           
=======================================
  Files         520      520           
  Lines       56405    56407    +2     
=======================================
+ Hits        16058    16060    +2     
  Misses      39081    39081           
  Partials     1266     1266           

Flag	Coverage Δ
.	23.74% <ø> (ø)
.-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	49.39% <ø> (+0.05%)	⬆️
.-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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