API v2 get customer

[c8y3]

Implement endpoint GET /api/v2/manage/customers/{identifier} to get the user profile.

• return 200 in case of success
• return a field in case of success
• create method in the business layer
• return 404 when the customer is not found
• return 403 when the user has no permission to read customers
• check returned object is uniform with other endpoints
• deprecate GET /manage/customers/<int:client_id>
• openapi: document endpoint
• openapi: deprecate GET /manage/customers/<int:client_id>