Add missing API: HttpSessionState.Mode

[Clounea]

Summary

When I try the IHttpModule and HttpApplication in the adapter, I found that HttpSessionState.Mode is used in my module(in Sharepoint Online) but it is not supported.

Motivation and goals

HttpSessionState.Mode is used in my module(in Sharepoint Online) but it is not supported. I wonder if there is plan to support the API.

In scope

HttpSessionState.Mode

Usage example:

context.Session.Mode != SessionStateMode.Off