Extract APIs (not implementation) from jmap-extensions module into their own maven module

[HoussemNasri]

Currently, the contact search engine, models, and related event bus infrastructure are located within the jmap-extensions module. This is not ideal as building anything on top of the contacts module requires depending on jmap-extensioons a completely unrelated and quite large module.

[chibenwa]

Add a new maven module:

tmail-backend/jmap/extensions-api

Relocate in there traits and interfaces that extensions-* projects needs.

definition of done

These projects shall not depend on tmail-backend/jmap/extensions