Add User Info to created tasks (#303)

This PR makes user information from JWT tokens available as part of a Task.

Included changes:
* Renamed `verify_token` to `call_if_agent`, since this function is specific to agent token verification
* Renames `is_authorized` to `is_agent`, since this function checks if the token is an agent
* Adds support for unmanaged nodes in `is_agent` (see #133 for information) 
* Saves the user information from the JWT token on task create as part of `TaskConfig`

Note, `TaskConfig` is what is provided to notification templates.  This enables Github issues and ADO work items to tie back to the user that created the task.

Note, while `upn` _usually_ means email for AAD user tokens.  If we were going to make use of the email address, we should perform a graph lookup based on the `oid`, but we're not.

src/api-service/__app__/onefuzzlib/user_credentials.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+#
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+from uuid import UUID
+
+import azure.functions as func
+import jwt
+from onefuzztypes.enums import ErrorCode
+from onefuzztypes.models import Error, Result, UserInfo
+
+
+def parse_jwt_token(request: func.HttpRequest) -> Result[UserInfo]:
+    """ Obtains the Access Token from the Authorization Header """
+
+    auth: str = request.headers.get("Authorization", None)
+    if not auth:
+        return Error(
+            code=ErrorCode.INVALID_REQUEST, errors=["Authorization header is expected"]
+        )
+
+    parts = auth.split()
+
+    if len(parts) != 2:
+        return Error(
+            code=ErrorCode.INVALID_REQUEST, errors=["Invalid authorization header"]
+        )
+
+    if parts[0].lower() != "bearer":
+        return Error(
+            code=ErrorCode.INVALID_REQUEST,
+            errors=["Authorization header must start with Bearer"],
+        )
+
+    # This token has already been verified by the azure authentication layer
+    token = jwt.decode(parts[1], verify=False)
+
+    application_id = UUID(token["appid"])
+    object_id = UUID(token["oid"]) if "oid" in token else None
+    upn = token.get("upn")
+    return UserInfo(application_id=application_id, object_id=object_id, upn=upn)