feat(authenticate-service): add filter to modify payload

[Vheissu]

Issue Link

#47

Types of changes

• New feature

Description

I needed to extend the returned payload from the plugin. The plugin only allows you to choose from a fixed number of payload options. By adding in a filter called simple_jwt_login_generate_payload, we can add new payload values to the generated JSON Web Token. I needed this to add a user avatar to the payload.

How has this been tested?

This is a minor addition and has been tested in a real-world application. An example of this filter being used can be found in this open source application here.

As the apply_filters API is being used to pass in the $payload variable and then return it, nothing happens by default unless the user implements their own filter hook.

Please note that I did not write any test cases for this, as the behaviour introduced into this change is related to a WordPress API function.

Checklist:

• My code is tested.
• My code follows Simple-JWT-login code style