Add admin endpoint for managing services.

[ksaaskil]

Adds the admin API, closes #376.

Endpoints

Get all services:

$ curl http://localhost:8888/services

Post a new service with name my-service:

$ curl -X POST http://localhost:8888/services/my-service -H "Content-Type: application/json" --data "@/path/to/openapi.json"

Note that this doesn't yet accept YAML.

Get OpenAPI schema for service with name my-service:

$ curl http://localhost:8888/services/my-service

TODO

• [ ] Tests 😁
• [ ] Use something more modern instead of express like koa or nest.js