[FR] REST API v2.1.0 first tasks endpoints

[Elise17]

This issue is about the implementation of the first tasks endpoints.

POST /api/v2/cases/{identifier}/tasks
GET /api/v2/tasks/{identifier}
DEL /api/v2/tasks/{identifier}

For each endpoint, the tasks are the following:

document the new API in the openapi documentation
implement automatic tests+code
deprecate the previous endpoint
replace the previous endpoints with the new ones in the web interface

This are the previous endpoints which will be deprecated by these:

In case_ioc_routes.py :

url : /case/tasks/add (old url) => POST /api/v2/cases/{identifier}/tasks
url : /case/tasks/{task_id} (old url) => GET /api/v2/tasks/{identifier}
url : /case/task/delete/{task_id} (old url) => DEL api/v2/tasks/{identifier}