Relayer packages to remove dependencies for adapters

[twsouthwick]

This change separates out the System.Web types into a single assembly that doesn't have any reliance on other dependencies. Any additional dependencies will be brought in by the extensions library which will only need to be added to the ASP.NET Core and ASP.NET Framework applications themselves (not any supporting libraries).

Fixes #131

[twsouthwick]

@davidfowl @Tratcher @mjrousos

I've updated the PR per our discussion this morning. There are a few types I'm including in the adapters libraries so they can potentially be used for 3rd party libraries (i.e. MessagePack support for session keys):

• ISessionState - container for session state
• ISessionSerializer - serializes ISessionState instances
• ISessionKeySerializer - serializes session key/value pairs

The following are public APIs that are shared services. As discussed, I'm using shared sources and they do not make sense in the adapter layer:

• JsonSessionSerializerExtensions: Registers JSON support for session
• JsonSessionSerializerOptions: Options for the json serializer
• SessionSerializerOptions: General options for session serializers
• ISystemWebAdapterBuilder: Builder type for registering adapter functionality/services
• RemoteAppOptions: Options for remote app information options

I don't think we want to have same type names but not actually same types in two assemblies, so these need to be renamed or shared in a 4th package (i.e. Microsoft.AspNetCore.SystemWebAdapters.Services that would target .NET 6+ and .NET 4.7.2). Thoughts?

[Tratcher]

When this ships we'll want to obsolete the old packages on nuget.