Refine Attribute Support

[michaelstaib]

This PR introduces new type attributes to force the type discovery to recognize a struct for instance as a ObjectType.

Also we have fixed the UsePagingAttribute and reworked the attribute base to include the annotated element and the descriptor context.

Last but not least we added the ExtendObjectTypeAttribute.

ISchema schema = SchemaBuilder.New()
    .AddType<StructQuery>()
    .AddType<StructQueryExtension>()
    .ModifyOptions(o => o.RemoveUnreachableTypes = true)
    .Create();

[ObjectType(Name = "Query")]
public struct StructQuery
{
    public string? Foo { get; }
}

[ExtendObjectType(Name = "Query")]
public class StructQueryExtension
{
    public string? Bar { get; }
}

will result in:

type Query {
    bar: String
    foo: String
}