Replace the BackendService Plugin by an injected method

[rhuille]

I / Remove fetchCollections method

BackendService has 2 methods:

• executePipeline
• fetchCollections

Only executePipeline is useful, fetchCollections should be removed

|| / Replace the store.subscribe by a store.watch on registerModule

https://github.com/ToucanToco/weaverbird/blob/master/src/store/backend-plugin.ts#L110

III / Inject the executePipeline as an action store

[WIP]

Technically, how should we do that ?