Support to inject a 'context augmenter' in DocumentsResource

In a downstream project, we need to be able to serve a link for a branch of a channel.

The downstream project has a customer channelManagerHstSiteProvider in DelegatingHstSiteProvider, but we somehow need to be able to inject knowledge into the context whether #getSite should return the master or whether a branch should be tried.

via the generic DocumentContextAugmenter, we can store extra logic on the HstRequestContext in a downstream project and use that later on during the request processing