fixing non-oneof inline struct handling in schemas

[balopat]

Fixes #1900, #1901.
And adds unit tests for schema generation + zero-type checking in skaffold fix (fixing a panic case).

[codecov-io]

Codecov Report

Merging #1904 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1904      +/-   ##
==========================================
+ Coverage   49.46%   49.47%   +0.01%     
==========================================
  Files         174      174              
  Lines        8036     8038       +2     
==========================================
+ Hits         3975     3977       +2     
  Misses       3683     3683              
  Partials      378      378

Impacted Files	Coverage Δ
pkg/skaffold/schema/v1beta1/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta2/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta5/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta6/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1alpha3/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1alpha5/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta4/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta3/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1alpha1/config.go	100% <ø> (ø)	⬆️
... and 3 more

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[balopat]

I just realized that with this change we have to add "oneOf" tag to all the oneOf fields in old schema versions as well - I'll do that soon.

[tejal29]

Few other test scenarios:

1. What if an Inline Struct is a combination of oneof and other fields? shd this result as an error
2. Inline Struct contains another Inline?

[balopat]

Few other test scenarios:

1. What if an Inline Struct is a combination of oneof and other fields? shd this result as an error

this should be validated by schema unit tests - they are not currently tested.

2. Inline Struct contains another Inline?

I would think this wouldn't work with schemagen yet, but haven't tried it - we should create an issue to explore/cover/validate against this edge case but doesn't belong to this PR.