Add function to return id_token (for Service Now)

[mryerse]

Some IDPs such as OpenAM can be configured to provide both an access_token and an id_token with the callback. This update provides a function that can be called to return the id_token instead of the access_token. The id_token contains a payload with claims related to the authenticated identity. This was required for API integration to Service Now, who told me their service cannot be configured to accept an access_token.

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[mryerse]

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[mryerse]

In my testing this works as expected since the id_token follows same expiration as access_token. Perhaps there's a better way to handle the situation, but if not then I hope this addition helps some people out.

[erickoledadevrel]

Thanks for the submission! There are two existing samples that use id_token by just getting the whole token and pulling it out:

https://github.com/gsuitedevs/apps-script-oauth2/search?q=id_token&unscoped_q=id_token

You could have done the same for your use case, but adding a helper function sounds like a good idea. Please update those two other samples to use this new method and then I'll merge the PR.

[mryerse]

You're right, I learned a new way to search today. But I'll take your word that it's a good idea, and just updated the two samples.