Allowing HTTP CONNECT requests

### Is your feature request related to a problem? Please describe.
I am writing an HTTP proxy server using ASP.NET Core. To handle HTTP `CONNECT` requests I would like to upgrade the request to its underlying stream and pipe it directly to the destination server. However it seems that upgrading non-upgrade requests is not supported and will throw an exception instead.

### Describe the solution you'd like
Consider adding a `force` parameter like `IHttpUpgradeFeature.UpgradeAsync(bool force = false)` that will upgrade the connection regardless of the presence of the upgrade header. Or to maintain backward compatibility, add a separate feature interface that will do this.

### Additional context