Missing lambda-based value converter model building APIs for ElementTypeBuilder.HasConversion

ElementType.HasConversion has HasConversion accepting a value converter type, but no easier lambda-based overloads like we do on PropertyBuilder.HasConversion.

IIRC we maybe said we won't introduce these because expression lambdas are incompatible with the compiled model; however, we now have the ability to generate them (with LinqToCSharpSyntaxTranslator), so we should consider adding the overloads. Even if we don't support these overloads with the compiled model for 8.0, the situation would be the same as with PropertyBuilder.