Add HttpContext.Endpoint property

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

ASP.NET Core includes the [GetEndpoint and SetEndpoint extension methods](https://github.com/dotnet/aspnetcore/blob/00d0038f937f0059a847fde504649fe33ec935e0/src/Http/Http.Abstractions/src/Routing/EndpointHttpContextExtensions.cs#L13-L56) for working with endpoints and HttpContext.

I think endpoints have become such an intrinsic feature to ASP.NET Core that we should consider making it a real property on `HttpContext`. A property would be easier to use (no need to have the right namespace for the extension method) and feel better to use compared to a method.

### Describe the solution you'd like

Add `HttpContext.Endpoint` property.

### Additional context

_No response_

Start with src/Http/Http.Abstractions/src/Routing/EndpointHttpContextExtensions.cs and inspect GetEndpoint and SetEndpoint, then trace the HttpContext API they extend. Done means HttpContext exposes an Endpoint property with the endpoint access currently provided by those extensions, with behavior verified by the relevant project tests.

Written by the indexing model from the issue text.