Add placeholders for k8s deployment stages

[Warashi]

What this PR does:

This PR adds placeholder implementations for the Kubernetes deployment plugin.

Why we need it:

I want to clarify where to implement stages.

Which issue(s) this PR fixes:

Part of #4980

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 0% with 42 lines in your changes missing coverage. Please review.

Project coverage is 25.75%. Comparing base (5079ca7) to head (83ce771).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...app/pipedv1/plugin/kubernetes/deployment/plugin.go	0.00%	24 Missing ⚠️
...p/pipedv1/plugin/kubernetes/deployment/baseline.go	0.00%	6 Missing ⚠️
...app/pipedv1/plugin/kubernetes/deployment/canary.go	0.00%	6 Missing ⚠️
...pp/pipedv1/plugin/kubernetes/deployment/primary.go	0.00%	3 Missing ⚠️
...pp/pipedv1/plugin/kubernetes/deployment/traffic.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5690      +/-   ##
==========================================
- Coverage   25.76%   25.75%   -0.01%     
==========================================
  Files         476      480       +4     
  Lines       50976    51018      +42     
==========================================
+ Hits        13135    13141       +6     
- Misses      36843    36879      +36     
  Partials      998      998              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[t-kikuc]

that made sense

[ffjlabo]

LGTM