feat: Allow object as payload

[Oinkling]

Changed JWT::encode and JWT::jsonEncode so that they now accept an object as well as an array as their first argument.

As the result of JWT::decode is an object it only makes sense that the same object can be encoded again.

This also makes the use case of using JWT as a session manager more straight forward as you can just encode and decode the user object directly without array conversions.

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[bshaffer]

This looks great but it's missing a few tests - would you be able to write some? They could be very simple / copied over from https://github.com/firebase/php-jwt/blob/main/tests/JWTTest.php

[Oinkling]

I have added a single test that just uses an object payload instead of array.
As long as we're sure it can encode that everything else works exactly the same as an array payload, so any further test seemed redundant.