[feat] /projections implementation

[lgrd]

Need

We need to know available projections, and to manage them on a specific service.

Tasks

• [x] Specify all the IO inside the API yaml
• [ ] GET /services/{service}/projections
• [ ] POST /services/{service}/projections
• [x] GET /services/{service}/projections/{projection}. See #63 and #64
• [ ] DELETE /services/{service}/projections/{projection}