Add back HttpRequest.LogonUserIdentity

[TaoziZ03]

Summary

HttpRequest.LogonUserIdentity was removed few days ago in #510. But it is being actively used in our codebase. Please add it back.

Motivation and goals

1-2 paragraphs, or a bullet-pointed list. What existing pain points does this solve? What evidence shows it's valuable to solve this?

In scope

A list of major scenarios, perhaps in priority order.

Out of scope

Scenarios you explicitly want to exclude.

Risks / unknowns

How might developers misinterpret/misuse this? How might implementing it restrict us from other enhancements in the future? Also list any perf/security/correctness concerns.

Examples

switch (context.Request.LogonUserIdentity.AuthenticationType.ToUpper(CultureInfo.InvariantCulture))
{
......
}

IntPtr userToken = context.Request.LogonUserIdentity.Token