add: creation of 2 hooks for ITIL actors

[froozeify]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• This change requires a documentation update. (add: new ITIL actors hooks docdev#187 - To merge once this PR is accepted)
• I have added tests that prove my fix is effective or that my feature works.

Description

• Add the ability to change the ITIL Actor icon
• Gave the possibility to plugin to alter the count of Actors and to add (or alter) actors listing : POST_ITIL_ACTORS_LOAD and POST_ITIL_ACTORS_COUNT

Screenshots

• Hook::POST_ITIL_ACTORS_COUNT called by the section title to defined the number (2)
• Hook::POST_ITIL_ACTORS_LOAD called to virtually add the whatsapp user (with a custom actor icon to it)

[AdrienClairembault]

For extra context, this is needed for the whatapp plugin to create an anonymous ticket with a phone number (like we do for the mail collectors with the emails).

Personally I would prefer creating real users (with no profiles so they can't login) as it seem less hacky and allow all others features to work seamlessly (e.g. searching ticket by requester, which won't work here).