[Feature] Fetching features via API filtered by tag

[eneco-pepe-bawagan]

Description of Feature

Right now, features can only be fetched via the API at the following granularities:

1. all features
2. all features within a project
3. a specific feature given an id

We wish to fetch features at a granularity between 2 and 3, e.g. to fetch all features within a specific view/page of an app/website.

This could be provided by introducing a new level of organization or by allowing filtering by tag.