Add hook for custom processing of webhook data

[artfulrobot]

Organisations may need to do more with the webhook data, e.g. deal with consent options. There is no standard meaning attached to the consent options in iParl - everything is completely configuratble, therefore this extension cannot handle the data without local knowledge.

We should make a hook available so that the incoming webhook data, plus the contact found/created, plus the activity created are passed to that hook.