refactor(config): use viper.Unmarshal capabilities

[GeorgeMac]

This change is in reaction to a comment that went by in the last configuration refactor PR.

Currently, we do lots of viper.IsSet and viper.Get interrogation and extraction.
This PR changes the strategy to lean into viper.Unmarshal capabilities on the entire config.Config structure.
Along with a little sugar to aid in keeping the configuration files focussed.

To add a field to an existing configuration, you now simply amend the structure:

type CorsConfig struct {
	Enabled        bool     `json:"enabled" mapstructure:"enabled"`
	AllowedOrigins []string `json:"allowedOrigins,omitempty" mapstructure:"allowed_origins"`
+	AllowedMethods []string `json:"allowedMethods,omitempty" mapstructure:"allowed_methods"`
}

To set a default, you ensure the sub-configuration implements the defaulter interface.
In the setDefaults method you add your default like so:

func (c *CorsConfig) setDefaults(v *viper.Viper) []string {
	v.SetDefault("cors", map[string]any{
		"allowed_origins": "*",
+		"allowed_methods": "GET,PUT,POST",
	})

	return nil
}

To validate the resulting configuration is appropriate, the sub-configuration can implement the validator interface.

func (c *CorsConfig) validate() error {
	for _, method := range c.AllowedMethods {
		if _, ok := map[string]struct{}{
			"GET": struct{}{},
			"PUT": struct{}{},
			"POST": struct{}{},
		}[method]; !ok {
			return nil, fmt.Errorf("%q: %w", method, errValidationMethodNotAllowed)
		}
	}
	
	return nil
}

[codecov-commenter]

Codecov Report

Merging #1079 (f029ca3) into main (83469f2) will decrease coverage by 0.36%.
The diff coverage is 97.16%.

@@            Coverage Diff             @@
##             main    #1079      +/-   ##
==========================================
- Coverage   81.16%   80.80%   -0.37%     
==========================================
  Files          26       26              
  Lines        1954     1870      -84     
==========================================
- Hits         1586     1511      -75     
+ Misses        285      279       -6     
+ Partials       83       80       -3     

Impacted Files	Coverage Δ
internal/config/config.go	76.71% <91.30%> (-10.33%)	⬇️
internal/config/cache.go	100.00% <100.00%> (ø)
internal/config/cors.go	100.00% <100.00%> (ø)
internal/config/database.go	100.00% <100.00%> (ø)
internal/config/log.go	100.00% <100.00%> (+16.66%)	⬆️
internal/config/meta.go	100.00% <100.00%> (+27.27%)	⬆️
internal/config/server.go	100.00% <100.00%> (ø)
internal/config/tracing.go	100.00% <100.00%> (+54.54%)	⬆️
internal/config/ui.go	100.00% <100.00%> (ø)
... and 4 more

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[markphelps]

Amazing work! I wonder in the future if we could/should separate out 'warnings' into it's own step/interface so that it doesn't fall on the responsibility of the defaults 'step'? Not necessary at the moment I don't think given we only have 2 warnings.

Thank you for doing this!!! 🥇