fix: Detect unknown fields in invalid specs as OutOfSync

[jgwest]

There are a couple of spots in diff.go that are unmarshalling JSON resource data into k8s structs, but in doing so they are silently tossing (invalid) JSON fields that are not present in that struct. In general, this is due to the use of json.Unmarshal(...) (whose default implementation silently ignores extra fields) in conjunction with k8s structs that do not contain those extra fields.

The fix is:

• Use decoder := json.NewDecoder(strings.NewReader(string(data))) and decoder.DisallowUnknownFields() to detect unknown fields when remarshalling
• When applyPatching against a Kubernetes resource, indirectly apply the result of the scheme default functions to the target JSON, rather than directly marshalling the scheme function results to the k8s resource (which would cause those extra fields to be discarded)

See code comments for details.

Fixes argoproj/argo-cd#1787

[codecov-commenter]

Codecov Report

Merging #154 into master will increase coverage by 0.24%.
The diff coverage is 54.28%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   53.79%   54.03%   +0.24%     
==========================================
  Files          25       25              
  Lines        2400     2439      +39     
==========================================
+ Hits         1291     1318      +27     
- Misses        974      976       +2     
- Partials      135      145      +10     

Impacted Files	Coverage Δ
pkg/diff/diff.go	63.06% <54.28%> (-0.43%)	⬇️
pkg/health/health.go	55.40% <0.00%> (-0.60%)	⬇️
pkg/sync/sync_context.go	70.36% <0.00%> (+0.24%)	⬆️
pkg/cache/cluster.go	49.87% <0.00%> (+1.45%)	⬆️

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
1 Code Smell

No Coverage information
24.1% Duplication

[codecov-io]

Codecov Report

Merging #154 into master will increase coverage by 0.10%.
The diff coverage is 54.28%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   48.14%   48.25%   +0.10%     
==========================================
  Files          36       36              
  Lines        2858     2887      +29     
==========================================
+ Hits         1376     1393      +17     
- Misses       1339     1341       +2     
- Partials      143      153      +10     

Impacted Files	Coverage Δ
pkg/diff/diff.go	61.45% <54.28%> (-0.25%)	⬇️

---

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[alexmt]

Awesome solution @jgwest ! I did not think it is even possible to support unknown fields. Thank you!

LGTM