Make RouteAttribute non-inherited

[rynowak]

Fixes: #5529

Inheriting and looking for inherited route attributes will cause nothing
but trouble. We had a bug tracking what to do about this and we decided
to make it really clear that routes are not inherited.

Previously the attribute was marked as inherited, but we woulnd't look
for inherited routes.

[SteveSandersonMS]

Would be great to have an extra unit test in RouteTableTests to represent this.

Besides that, looks great!

[rynowak]

Added some tests

[pranavkm]

Make it sealed? I think you're allowed to redefine AttributeUsage on derived types so you could get in to the same problem.

[rynowak]

Hmm that's wierd that we didn't do this already. Since github didn't show your comment until after I merged, I'm going to add this to the API review item.

[gsnoff]

@rynowak @SteveSandersonMS One possible legitimate scenario with inherited route templates I could see, is when the API scheme is using API version in the URL path, and one would like to avoid duplicating the same [Route("api/v{version:apiVersion}/[controller]")] attribute in every versioned API controller in order to uphold the DRY principle.

Right now I’m having to resort to a IEndpointRouteBuilder.MapControllerRoute(...) call in my Startup class to configure something like that, which just isn’t that flexible when my controllers would have vastly different sets of action endpoints.

[rynowak]

This issue was related to Blazor. What you're suggesting with work fine with MVC.