Add IConnectionLifetimeFeature to Microsoft.AspNetCore.Connections.Abstractions

[davidfowl]

public interface IConnectionLifetimeFeature
{
    CancellationToken ConnectionClosed { get; }
    void Abort();
}

These should also be exposed on ConnectionContext

Related to: #1980

[muratg]

Putting this in 2.2 milestone for now (like the other bedrock items).

[KPixel]

I don't think that Abort should be exposed directly on ConnectionContext.

As it is, ConnectionContext has a very clean API: The Transport which is the core reason why this type exists and two extensibility properties (Features and Items).

Anything else would feel too specific.
(I can give a pass to ConnectionId because it is a simple property and it is useful for logging.)

By the way, should Abort be async-capable?

[davidfowl]

I don't think that Abort should be exposed directly on ConnectionContext.

I think it needs to be. It's way to directly communicate with the underlying transport skipping all indirections (the Transport pipe might have been replaced).

By the way, should Abort be async-capable?

I don't think so. It's a signal, fire and forget.

I updated the interface.

[KPixel]

This interface will be available through the Features property. And that's what that property is there for.

I just don't understand why it also needs to be "hard-coded" into ConnectionContext.

[davidfowl]

Because every connection needs the ability to be aborted and that is a distinct feature from completing the transport.