Added parsing of id_token, if present in access token response

[colinhumber]

Adds support for parsing of id_token, if present, when fetching an access token. Details on why this is useful is in #668, specifically to support Single Sign Out with a redirect URI.

[phimage]

Hi,

Ok we just decode it if present and store it in memory into OAuthSwiftCredential

No need to store it ? in keychain for instance with other credential object informations

There is support of Codable and old NSCoder too

ps: I could merge anyway but could wait

[colinhumber]

I missed encoding/decoding with Codable and NSCoder support on OAuthSwiftCredential. I'll add that in.

[MikeSpelling]

Is this still being looked at? I also need the id_token for single sign-out so would appreciate the merge!

[MikeSpelling]

Any reason this is still unmerged?

[ramden]

Same question, why still unmerged? This would be very usable, because the library seems to lack proper logout and id_token is needed to acheive that via manually requests