refactor: new validation rule interface

[mattwelke]

Description

New interface to help with working with rule names in a consistent way across the ecosystem.

Plugins would be required to implement it for easy integration with programs like validatorctl. They can do that by embedding validationrule.ManuallyNamed or validationrule.AutomaticallyNamed into their rule struct and implementing the required methods. If they embed validationrule.AutomaticallyNamed, they don't need to implement SetName. Example:

type MyRule struct {
	validationrule.ManuallyNamed `json:",inline"`

	...
}

They should also assert that their rule implements the interface. Example:

var _ validationrule.Interface = (*MyRule)(nil)

For rules that are manually named, it is expected that only pointers to rules can implement the interface because of the pointer receiver of implementations of SetName. Example:

func (r *MyRule) SetName(name string) { ... }

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Files	Patch %	Lines
pkg/validationrule/validation_rule.go	0.00%	5 Missing ⚠️
pkg/validationresult/validation_result.go	0.00%	2 Missing ⚠️

@@            Coverage Diff             @@
##             main     #399      +/-   ##
==========================================
- Coverage   54.02%   53.78%   -0.24%     
==========================================
  Files          21       22       +1     
  Lines        1131     1136       +5     
==========================================
  Hits          611      611              
- Misses        452      457       +5     
  Partials       68       68              

Files	Coverage Δ
pkg/validationresult/validation_result.go	73.58% <0.00%> (ø)
pkg/validationrule/validation_rule.go	0.00% <0.00%> (ø)

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[mattwelke]

Made some changes after review.

• Renamed "allows manual name" to "requires name"
• Kept interface named Interface instead of changing it to ValidationRule to avoid package stutter (validationrule.Interface instead of validationrule.ValidationRule)