Feature Request - Add id_token support to OAuthSwiftCredential

[MikeSpelling]

Description:

In order to support single-sign out with a redirect endpoint you need access to the original id_token received during authentication (https://identityserver4.readthedocs.io/en/latest/endpoints/endsession.html#refendsession).
It seems like this should be easy enough to add to the OAuthSwiftCredential object as an optional String.
Around line 122 of OAuth2Swift for example (where the access_token is set) the id_token could also be added?

OAuth Provider? (Twitter, Github, ..):

Custom - using our own Identity Server

OAuth Version:

• Version 1
• Version 2

OS (Please fill the version) :

• iOS :
• OSX :
• TVOS :
• WatchOS :

Installation method:

• Carthage
• CocoaPods
• Swift Package Manager
• Manually

Library version:

• head
• v2.1.0
• v2.0.0
• v1.4.1
• other: (v2.2.0)

Xcode version:

• 11.4 (Swift 5.2)
• 11.x (Swift 5.1)
• 10.x (Swift 5.0)
• other: (Xcode 12.5.1, Swift 5.4)
• objective c

[colinhumber]

I've added support for this in #679