Add `platformSelf` function to -auth package

[dbouwman]

from confluence...

/platformSelf Endpoint: /sharing/rest/oauth2/platformSelf (POST only)

This call returns token after performing checks made by /validateAppAccess call - i.e it returns app specific token of calling user only if user has access to the app and the encrypted cookie is valid.

Expiration of the token received matches the platform cookie's expiration.

Request Headers:

• Origin - origin representing the origin of the app making this request
• Referer – Referer of the request. Either origin or referer have to be provided and these have to be from the same domain as that of arcgis.com, esri.com for AGOL and the Enterprise Portal domain for on-prem.
• X-Esri-Auth-Client-Id – ClientID of the app that is making this self request to fetch the user information from the platform cookie if exists. This is only allowed for some of the Esri apps.
• X-Esri-Auth-Redirect-Uri – Redirect Uri of the app that is making this self request to fetch the user information from the platform cookie if exists.

Response Type: json

Response: { “username“: “<username>”, token”: “asdasdfasdf” }

If the cookie does not exist, following error is returned.

{ code: 400, messageCode: "OAUTH_0066", message: "No signed in user found.", “details”:[]}

[dbouwman]

added... but there is an issue w/ the credential fetch param we are still addressing

[gavinr]

@dbouwman can this one be closed?