Support EasyAuth

[davidebbo]

To make it possible for the Functions to perform authorization, we should pass the various auth claims into the context object.

Idea is to allow users to specify a new "user" auth level value to indicate that a function requires an EasyAuth validated identity.

[mathewc]

Retitling

[mattchenderson]

Update here: investigated with @fabiocav on Friday and confirmed that .NET functions can just rely on the ClaimsPrincipal, per the code below. Other languages would have to rely on the headers, and that is not code we should make users write. Would be nice to expose as part of the context object or similar.

using System.Net;
using System.Threading;
using System.Security.Claims;

public static void Run(HttpRequestMessage req, TraceWriter log)
{
    if (!Thread.CurrentPrincipal.Identity.IsAuthenticated)
    {
        log.Info("Not authenticated");
        return;
    }

    ClaimsIdentity identity = (Thread.CurrentPrincipal as ClaimsPrincipal)?.Identity as ClaimsIdentity;
    foreach (var claim in identity.Claims)
    {
       log.Info($"{claim.Type} = {claim.Value}");
    }
}

[mathewc]

@mattchenderson I assume both C# and F# will now work, since we just merged the new F# model. For Node, can you give some examples of what needs to be exposed? Is it just things like context.identity.isAuthenticated and context.identity.claims? I.e. can we simply translate the identity into a json object?

Also, we'd then start exposing one more enum value for our existing authLevel property: "user", to indicate that a function requires an authenticated identity. I've already made a provision for that in our enum in anticipation of this :)

[fabiocav]

@mathewc we may also want to consider making the identity information available as named inputs. Thoughts on that?