feature: Custom Formatters in ReactiveValidationObject

[worldbeater]

What kind of change does this PR introduce?

Currently, our INotifyDataErrorInfo implementation doesn't support custom formatters. The support for custom validation text formatters might be useful in cases when one needs to implement localization in their application via passing resource keys to ValidationRule calls instead of validation messages.

What is the current behavior?

Currently, there is no way to customize messages emitted by INotifyDataErrorInfo.

What is the new behavior?

Now, in order to customize the messages emitted by INotifyDataErrorInfo, one needs to either pass an instance of a custom IValidationTextFormatter<string> to ReactiveValidationObject constructor, or to register a default IValidationTextFormatter<string> globally via Locator.Current.RegisterConstant, as documented in #153

What might this PR break?

Nothing, this change shouldn't be breaking.

[codecov]

Codecov Report

Merging #154 into main will increase coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
+ Coverage   62.14%   62.34%   +0.20%     
==========================================
  Files          16       16              
  Lines         906      911       +5     
==========================================
+ Hits          563      568       +5     
  Misses        343      343              

Impacted Files	Coverage Δ
Helpers/ReactiveValidationObject.cs	97.67% <0.00%> (+0.30%)	⬆️

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