Allow ASP.NET Core session APIs to be used with remote session state

[mjrousos]

Once users have moved some code from their ASP.NET app to the ASP.NET Core one, it may be valuable, as a next step, to be able to update that code to use "real" ASP.NET Core APIs instead of adapters.

In order to enable that, it may be useful to have ISessionStore and ISession implementations that allow ASP.NET Core session APIs to be used while still storing session items in remote session storage for interoperability with other endpoints that are still on ASP.NET.

The user would still have to use the ASP.NET Core session API (that's the whole point of this migration) so their code would need to change to only read/write byte[]s and any code in other parts of the solution that works with those session items would also need to be updated to work with byte[]s, but those changes would be necessary sooner or later as part of migration anyhow. Having the ability for those session items to be shared between the two apps just enables some parts of the solution to be more completely migrated even before the entire app is running on Core.