feat: add Client.ReloadDeclarativeConfig()

[pmalek]

This PR adds possibility to send declarative configs to Kong via /config.

It also makes Client.NewRequestRaw() more "flexible" ( I guess? ) by checking the type of the accepted argument and calls json.Marshal() only when the provided argument is not string, []byte or io.Reader.

---

Unfortunately we cannot accept proper types as parameters because deck already imports go-kong so using e.g. KongRawState or file.Content is not possible because that would cause a circular dependency.

[codecov-commenter]

Codecov Report

Base: 53.31% // Head: 52.85% // Decreases project coverage by -0.45% ⚠️

Coverage data is based on head (3e8fc96) compared to base (9a0b9ec).
Patch coverage: 29.11% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
- Coverage   53.31%   52.85%   -0.46%     
==========================================
  Files          46       47       +1     
  Lines        4183     4255      +72     
==========================================
+ Hits         2230     2249      +19     
- Misses       1478     1525      +47     
- Partials      475      481       +6     

Flag	Coverage Δ
2.1.4	39.83% <29.11%> (-0.24%)	⬇️
2.2.1.3	48.95% <29.11%> (-0.11%)	⬇️
2.2.2	39.83% <29.11%> (-0.24%)	⬇️
2.3.3	39.83% <29.11%> (-0.24%)	⬇️
2.3.3.4	48.95% <29.11%> (-0.11%)	⬇️
2.4.0	39.83% <29.11%> (-0.24%)	⬇️
2.4.1.3	48.95% <29.11%> (-0.11%)	⬇️
2.5.1	39.83% <29.11%> (-0.24%)	⬇️
2.5.1.2	48.95% <29.11%> (-0.11%)	⬇️
2.6.0	39.83% <29.11%> (-0.24%)	⬇️
2.6.0.2	48.95% <29.11%> (-0.11%)	⬇️
2.7.0	39.83% <29.11%> (-0.24%)	⬇️
2.7.0.0	48.95% <29.11%> (-0.11%)	⬇️
2.8.0	39.83% <29.11%> (-0.24%)	⬇️
2.8.2.2	48.95% <29.11%> (-0.11%)	⬇️
3.0.1	39.17% <29.11%> (-0.23%)	⬇️
3.0.1.0	49.49% <29.11%> (-0.12%)	⬇️
3.1.0	39.17% <29.11%> (-0.23%)	⬇️
3.1.0.0	49.49% <29.11%> (-0.12%)	⬇️
community	39.83% <29.11%> (-0.24%)	⬇️
enterprise	50.15% <29.11%> (-0.13%)	⬇️
integration	52.85% <29.11%> (-0.46%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
kong/config_service.go	0.00% <0.00%> (ø)
kong/test_utils.go	47.50% <31.25%> (-5.91%)	⬇️
kong/request.go	65.71% <80.00%> (+10.15%)	⬆️
kong/client.go	60.57% <100.00%> (+0.19%)	⬆️

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