Add in filter to modify generated JWT payload

[Vheissu]

Is your feature request related to a problem?

My problem was I needed to add additional fields (in my case, it was an ACF image field for an avatar) to the payload.

Describe the solution you'd like

A simple apply_filters call inside the authentication service allows us to modify it before it is returned.

A pull request for this change has already been submitted here: #48