sidedata: add a 'side-data' repository feature and use it Most code don't really care how sidedata support is added, but it needs to know if it is present. To achieve this. we use the `repo.features` attributes with a new dedicated features". Having such centralised information is more robust and will help us to remove the sidedata requirements in a later changesets. Differential Revision: https://phab.mercurial-scm.org/D10617