[connectors-sdk] Add Incident model to the sdk

[maximerafaillac]

Use case

The connectors-sdk currently misses the following model, which are needed for future connectors:

• Incident

Current Workaround

Currently, developers must use classes provided by pycti, which do not provide validation or automatic ID generation (not DRY and error prone).

Proposed Solution

Add the mentioned model to the SDK.