Extend property covariance into the internal model.

[JeremySkinner]

Makes TProperty covariant within the internal model's IValidationRule<T,TProperty> interface.

This would resolve #1711 and related scenarios where the property type may be an inheritor of the type defined in a rule extension method.

This would introduce several breaking changes. Specifically:

• IValidationRule.Current would need to be IRuleComponent<T,TProperty> rather than RuleComponent<T,TProperty>
• IRuleComponent<T,TProperty>.CustomStateProvider would become write-only
• IRuleComponent<T,TProperty>.SeverityProvider would become write-only
• GetErrorMessage would not be exposed on IRuleComponent<T,TProperty>

Making various properties write-only is potentially problematic, but in theory users should only ever be setting these, and only the library's internals need to get them. But I expect there will be someone out there who relies on this for something nonstandard.

MessageBuilders still need some work. The quick fix is to not expose TProperty on the MessageBuilderContext and instead go back to exposing this as a non-generic object. This isn't ideal, so see whether there's a better way around this that still allows the MessageBuilder's getter to be exposed.