Add ToResponse derive implementation

[juhaku]

Add new ToResponse derive attribute macro. This can be used alternatively to generate path response. It support all the attributes which are supported in tuple response syntax to further configure the generated response. By default the name of the struct which is decorated with ToResponse macro will be used as a name for the response and as response type as well.

To define simple response component where name of the repsonse and schema must be same one can use named field struct or enum without content attributes.

 #[derive(utoipa::ToSchema, utoipa::ToResponse)]
 struct Person {
     name: String,
 }

To define multiple contents for single response one must use enum style definition.

 #[derive(utoipa::ToResponse)]
 enum Foo {
   Foo(#[content("type")] Foo),
   Bar(#[content("type")] Bar),
 }

To define Vec or Option response the new type pattern must be used.

 #[derive(utoipa::ToResponse)]
 struct MyResponse(Vec<String>);

Resolves #412