calls API: add conversation ID

In some scenarios involving a mobile endpoint, API events don't allow to distinguish the two scenarios:

1. User A receives a call on both endpoints phone and mobile
2. User A receives two calls on an phone

Proposition: add "conversation_id" that would be identical for both calls in scenario 1 and different in scenario 2

Example:

• User A has a mobile app + a physical phone

• User A receives a call

• User A is notified of two incoming calls

Expected:

• User A is notified on only one incoming call