Support making nonce nullable

[FelipeBuiles]

AppAuth-iOS worked correctly with AWS Cognito up until its 0.92 release, later they added support that added support for a nonce parameter. The issue lies in the inabilty of Cognito to handle the nonce parameter (mentioned in their forums here).
There is a comment on the OIDAuthorizationRequest.h file from AppAuth-iOS that mentions something about it being nullable:

    @param nonce String value used to associate a Client session with an ID Token. Can be set to nil
        if not using OpenID Connect, although pure OAuth servers should ignore params they don't
        understand anyway.

The idea here is to add a new optional param useNonce that when set to false, tells the iOS implementation to not generate a nonce attribute automatically. This only happens on the iOS implementation, Android does not have this issue.

[kadikraman]

This seems a bit fiddly, I'd prefer to just add it to the end as the android-only method above

nativeMethodArguments.push(useNonce);

[kadikraman]

Hi @FelipeBuiles thanks a lot for the PR!

Since this is AWS Cognito specific, would you be able to add a config example (something like this).

Could you also please add the new parameter to the config.

[dreyks]

actually i have this same issue with Dropbox API so I have high hopes for seeing that merged

[dreyks]

ugh... tried @FelipeBuiles fork, now getting unknown field 'code_challenge'. FFS dropbox, why not follow the RFC?!

So this PR alone won't help me. Also need to turn off PKCE as described in openid/AppAuth-iOS#305 (comment)

[dreyks]

nvm. Dropbox refuses to redirect to urls not starting from https in responseType code. And token is not supported by AppAuth so I have to find another OAuth library

[nol13]

same issue moving from 2.4.1, so far so good with this fix.

Not cognito and possibly could update a config somewhere in back-end, but this is working for us.

might depend on our setup but the experience was the redirect not working on first attempt vs. a complete failure to authenticate.

[kadikraman]

Thanks for your help! This has been published in v4.1.0.