[SDK-3144] Add user property to Credentials

[adamjmcgrath]

Changes

Added the user property to Credentials
Exposed a couple of methods on the Jwt helper to support this

Usage

authentication
    .login("info@auth0.com", "a secret password", "my-database-connection")
    .start(object: Callback<Credentials, AuthenticationException> {
        override fun onSuccess(credentials: Credentials) {
            println(credentials.user.name)
        }
    })

I didn't make it a Delegated Property because I couldn't get Gson to deserialize it (see https://stackoverflow.com/a/57604781)

Testing

Login to the sample app, note the new "Hello YOUR_NAME" greeting.

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines

• I have read the Auth0 Code of Conduct

• All existing and new tests complete without errors

[poovamraj]

👍