Fix skaffold filter to handle multiple yaml documents

[briandealwis]

Fixes: #4826,
Related: #4827, #4732

Description
As discovered by @doriandekoning in #4826, the new Helm debug support does correctly handle the situation where a Helm chart may have multiple resources.

This PR does the following:

• it introduces a manifest.Load() function to create a ManifestList from a YAML input leveraging the k8s.io/apimachinery's YAMLReader.
• filter now uses manifest.Load() to parse the manifests proferred on stdin
• adds a test for debug to verify that it applies to multiple manifests

[codecov]

Codecov Report

Merging #4829 into master will increase coverage by 0.01%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##           master    #4829      +/-   ##
==========================================
+ Coverage   71.72%   71.73%   +0.01%     
==========================================
  Files         353      353              
  Lines       12099    12108       +9     
==========================================
+ Hits         8678     8686       +8     
- Misses       2775     2776       +1     
  Partials      646      646              

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/filter.go	26.82% <0.00%> (+0.63%)	⬆️
pkg/skaffold/kubernetes/manifest/manifests.go	91.48% <80.00%> (-3.11%)	⬇️

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