add suggestion protos and hook up with Event API

[tejal29]

Fixes #4331

Add Suggestion to event API.
In this PR,

1. Add Suggestion to the ErrDef. Suggestion represent an action for a user to perform to recover from an error.
   Right now Suggestion includes:
   • SuggestionCode uniquely identifies a suggestion.
   • Action describes the action or step to recover.
     In future it can contain some metadata like if this message is kubernetes specific etc.

[codecov]

Codecov Report

Merging #4364 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4364      +/-   ##
==========================================
+ Coverage   71.74%   71.76%   +0.01%     
==========================================
  Files         324      324              
  Lines       12507    12532      +25     
==========================================
+ Hits         8973     8993      +20     
- Misses       2966     2969       +3     
- Partials      568      570       +2     

Impacted Files	Coverage Δ
pkg/skaffold/errors/errors.go	81.48% <100.00%> (+5.29%)	⬆️
pkg/skaffold/event/event.go	90.86% <100.00%> (+0.27%)	⬆️
pkg/skaffold/debug/transform_nodejs.go	79.62% <0.00%> (-1.24%)	⬇️
pkg/skaffold/deploy/kubectl.go	67.10% <0.00%> (-0.68%)	⬇️
pkg/skaffold/deploy/kustomize.go	77.05% <0.00%> (-0.66%)	⬇️
pkg/skaffold/config/util.go	70.71% <0.00%> (-0.55%)	⬇️
pkg/skaffold/debug/debug.go	45.16% <0.00%> (ø)
pkg/skaffold/debug/transform.go	88.77% <0.00%> (ø)
pkg/skaffold/deploy/transformations.go	100.00% <0.00%> (ø)
...affold/kubernetes/portforward/kubectl_forwarder.go	63.41% <0.00%> (+2.43%)	⬆️

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[nkubala]

small nits but LGTM