Support excluding selected object properties/parameters from validation

[oroztocil]

Currently, the new shared validation support (used in Minimal APIs and Blazor) does not have a mechanism for declaring that some object properties of a validated type should be skipped (i.e. the validator should not recurse into validating their types and their nested properties).

We should add an opt-out attribute similar to the ValidateNever attribute from MVC.

Name of the attribute is to be determined. We might want to avoid conflict with ValidateNever. Notably, SkipValidation is used as a property in JsonWriterOptions.

Proposed features:

• When user decorates a property of a validated type with the attribute, the validator skips that property during validation.
• When user decorates a parameter of a validated method with the attribute, the validator skips the parameter during validation.
• [TBD] When user decorates a type with the attribute, the validator skips all properties and parameters of that type during validation. (ValidateNever supports this.)
• The source generator skips creating validation code for a type/property/parameter that is only present in the assembly as decorated with the attribute. (If there is at least one use that is not skipped from validation, the code must be generated.)

[FelipeCostaGualberto]

Currently I use Blazor's ObjectGraphDataAnnotationsValidator's approach: you should decorate with [ValidateComplexType] the properties you want the validator to transverse. I think it works great.

I don't like the current approach where you decorate the root model and then all of its properties are validated, but that's just my opinion.