Return a unix timestamp for expiresIn

[cranberyxl]

Changes

You can only return JSON primitives over the bridge, the Foundation Date comes back as null for this reason.

Testing

Look at the credentials returned from CredentialsManager via the bridge.

const auth = new Auth0(...) 

auth.credentialsManager.getCredentials().then(console.log)

• 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
• All existing and new tests complete without errors
• All active GitHub checks have passed

[poovamraj]

That's a great catch! I have added a suggestion. Once it is merged, we can add this to the next release. Thanks a lot!