[feat] /projections implementation

[lgrd]

Need

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

Tasks

• Specify all the IO inside the API yaml
• GET /services/{service}/projections
• POST /services/{service}/projections
• GET /services/{service}/projections/{projection}. See [feat] /services/{service}/projections/{projection} implementation (1/2) #63 and [feat] GET /services/{service}/projections/{projection} implementation (2/2) #64
• DELETE /services/{service}/projections/{projection}