Support SNI to enable multiple HTTPS hosts on single server

[DamianEdwards]

There are three parts to this:

1. Work with corefx to define the SslStream API: dotnet/corefx#24553
2. Expose the new configuration in Kestrel's Https APIs.
3. Expose some form of this in Kestrel's config file format

@Tratcher @muratg

[Tratcher]

See https://github.com/dotnet/corefx/issues/24553, #2103

[JoshCollinsMSFT]

Assuming dotnet/corefx#24553 makes it in the .NET Core 2.1 release, will the next release of KestrelHttpServer have support for SNI?

[Tratcher]

That's the idea, will see how it goes.

[Eilon]

@DamianEdwards / @shirhatti / @Tratcher - is this critical for Preview2?

[Tratcher]

Yes. We're trying to take a new CoreFx build today to unblock it.

[Eilon]

Got it, thanks!