feat: Rewrite configloader

[irvinlim]

Partially addresses #28.

Rewrite the configloader package to avoid using viper.Viper, and use mergo instead. Added more tests.

[codecov]

Codecov Report

Merging #32 (43dad2a) into main (78a56ed) will decrease coverage by 0.02%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   49.05%   49.03%   -0.03%     
==========================================
  Files         160      159       -1     
  Lines        7951     7940      -11     
==========================================
- Hits         3900     3893       -7     
+ Misses       3864     3862       -2     
+ Partials      187      185       -2     

Impacted Files	Coverage Δ
pkg/runtime/controllercontext/context_configs.go	16.00% <ø> (ø)
pkg/runtime/configloader/manager.go	71.25% <53.84%> (ø)
pkg/runtime/configloader/configmap_loader.go	81.72% <100.00%> (ø)
pkg/runtime/configloader/defaults_loader.go	62.16% <100.00%> (ø)
pkg/runtime/configloader/secret_loader.go	69.23% <100.00%> (ø)
.../runtime/controllercontext/mock/context_configs.go	79.48% <100.00%> (+5.06%)	⬆️

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