API v2 paginated assets

[c8y3]

Implement endpoint GET /api/v2/cases/{case_identifier}/assets to get a paginated list of assets:

• successful GET should return 200
• in case the case is not found, return 404
• field data should be present
• fields data should be [] when there are no assets
• field total should return 1 when there is an asset
• it should accept page, per_page parameters
• it should accept order_by and sort_dir parameters
• fields last_page, current_page and next_page should be present in result
• introduce a method at the business level rather than going directly to the database
• use object PaginationParameter
• check returned object is uniform with other endpoints, use the correct schema object
• deprecate GET /case/assets/list
• openapi document GET /api/v2/cases/{case_identifier}/assets
• openapi deprecate GET /case/assets/filter

[c8y3]

Note "GET /case/assets/filter" was not documented in the openapi document, so no need to deprecate it there.