Add support for passwordless login [SDK-1940]

[sanoopps]

Describe the problem you'd like to have solved

The SDK currently does not support passwordless login method i.e. exchanging an OTP obtained from /passwordless/start call for tokens.

Describe the ideal solution

Ideally, I suppose the SDK would expose an auth API like below that supports exchanging OTP obtained via either email or sms -

AuthRequest request = auth.exchangeOtp("me@auth0.com", "123456", "email")
        .setScope("openid contacts");
try {
    TokenHolder result = request.execute();
} catch (APIException exception) {
    // api error
} catch (Auth0Exception e) {
    // request error
}

AuthRequest request = auth.exchangeOtp("0123456789", "123456", "sms")
        .setScope("openid contacts");
try {
    TokenHolder result = request.execute();
} catch (APIException exception) {
    // api error
} catch (Auth0Exception e) {
    // request error
}

Alternatives and current work-arounds

An alternative is to directly call the Auth API like any other HTTP call and not use this SDK.

Additional context

If you are happy to accept contributions, I have a branch with changes (not complete yet) that I can tag here.

[lbalmaceda]

I think it's a valid ask. But it would require more than just the OTP exchange, as the /passwordless/start endpoint is also missing. Out of curiosity, on what type of client/app are you using this SDK? and why would you prefer that over using the Auth0 hosted solution?

[sanoopps]

We're not using the Java SDK. We do use the auth0.js SDK though.

Yes, /passwordless/start is also missing. The reason I asked specifically about exchanging OTP is that we have a Java server side application interfacing with Auth0 for token issue and was looking at extending it to handle OTP exchange as the first step to build a passwordless login solution. My curiosity led me to this SDK and I was wondering why this functionality was missing in the SDK. That's all. :)

[lbalmaceda]

@sanoopps Ok, no problem. I'm tracking this feature request internally as SDK-1940.