[FR] REST API v2.1.0 first assets endpoints

[Elise17]

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

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

For each endpoint, the tasks are the following:

document the new API in the openapi documentation
implement automatic tests+code
deprecate the previous endpoints
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_assets_routes.py :

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