Introduce SameSiteMode.Unspecified equivalent in 3.x

[brettsam]

It looks like samesite=lax is not present by default in ASP.NET Core 3.0. See test RawScriptResultTests.AddsHttpCookie() -- it has been removed. Need to evaluate.

[brettsam]

This changed from 2.2 -> 3.0 -> 3.1. The value here went from samesite=lax to blank, to samesite=none.

• Link to test in dev branch: https://github.com/Azure/azure-functions-host/blob/dev/test/WebJobs.Script.Tests/Binding/ActionResults/RawScriptResultTests.cs#L83.
• 3.0 announcement change: [Announcement] Some Cookie SameSite defaults changed to None dotnet/aspnetcore#8212
• Bug in 3.0 (which prompted a change in 3.1): SameSite.None for cookies is ignored dotnet/aspnetcore#12125

[mhoeger]

The linked test actually represents an invalid case :) there will never be CookieOptions where "SameSite" is not set, because it is always set with a default value of "None" (code here)

What we need to introduce with V3 is the ability to set a value of "ExplicitNone", where this adds a value of "SameSite=None", and change the current v3 to treat SameSite=None as unspecified