Make Projects plugin independent of SDK implementation

Currently, the Projects plugin depends on the plugin-sdk/implementation module by means of using a number of static Utils classes. These Utils should be made available as Spring-injectable services, with an API (interface) defined in the plugin-sdk/api module. Once this dependency is detangled, the plugin will only rely on a stable API, and it should be possible to, upon separating the SDK from the dashboard, migrate the plugin into the WPM project.