Add external validator support

[DamianEdwards]

Replaces #67 with a revised external-validator implementation targeted for the 0.11.0 milestone.

Summary

• Adds IValidate<TTarget> and IAsyncValidate<TTarget> for external validators
• Resolves validators from the IServiceProvider already accepted by MiniValidator
• Supports single and IEnumerable<> validator registrations without adding a DI package dependency to the core library
• Handles nested objects and enumerable elements with external validators
• Caches delegates without capturing validator instances, preserving transient/scoped behavior
• Adds documentation and unit coverage for the new API

Validation

• dotnet test tests\MiniValidation.UnitTests\MiniValidation.UnitTests.csproj -f net8.0 --nologo --verbosity minimal
• dotnet build src\MiniValidation\MiniValidation.csproj -f netstandard2.0 --nologo --verbosity minimal