[v3] Add the Kptfile struct to render.

[yuwenma]

Related: #5673

Description

There are two approaches to read Kptfile. One is via templating which we can add comments to help users understand the Kptfile. The other is via go struct which is more accurate and less error-prone. Considering the reliability of the Kpt deprecation policies (the api schema is not expected to change and if it is changed it will be backward compatible. If not backward compatible, the deprecation has a year-long period), go struct is used here.

[codecov]

Codecov Report

Merging #5940 (df4817a) into master (fafdcd7) will decrease coverage by 0.09%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #5940      +/-   ##
==========================================
- Coverage   70.67%   70.58%   -0.10%     
==========================================
  Files         454      455       +1     
  Lines       17395    17427      +32     
==========================================
+ Hits        12294    12300       +6     
- Misses       4195     4219      +24     
- Partials      906      908       +2     

Impacted Files	Coverage Δ
pkg/skaffold/render/kptfile/pipeline_type.go	0.00% <0.00%> (ø)
pkg/skaffold/render/renderer/renderer.go	41.17% <44.44%> (-8.83%)	⬇️
pkg/skaffold/event/v2/event.go	74.03% <0.00%> (+0.12%)	⬆️
pkg/skaffold/runner/v1/dev.go	73.11% <0.00%> (+0.12%)	⬆️
...skaffold/kubernetes/debugging/container_manager.go	50.98% <0.00%> (+0.98%)	⬆️
...affold/kubernetes/portforward/forwarder_manager.go	50.00% <0.00%> (+1.11%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fafdcd7...df4817a. Read the comment docs.