indicate access restrictions/privacy information about a post and add implementer hints to respect them

cf https://github.com/aaronpk/Quill/issues/143 and https://chat.indieweb.org/dev/2022-02-28#t1646077696840800 based on some comments by @fluffy-critter

Clients (especially those that allow to post responses) should clearly indicate if a shown post is non-public and if responses are posted should direct those to be private too or at least clearly warn about them being public responses to a private post and that the user take necessary care to not reveal information. Similar concerns apply to generated reply contexts.

for this, clients need to be able to know that a post was private/access-restricted in the first place, so this needs to be exposed in the first place. This can be tricky depending on the specific implementations. Some ideas:

a) servers know if they used any authenticated method to fetch the feed content - but without extra info on the posts they can't tell if the post was restricted or not (if they fetch the post permalinks, they could try unauthed first and thus tell)

b) the concept of an audience as a [property of a post](https://indieweb.org/audience) has been about. If private posts mark this up, microsub readers can pass this through. There is little established about this though.

Start with the Microsub specification, then review Quill issue 143 and the linked IndieWeb discussion for the proposed privacy and audience signals. Done means the specification defines how clients identify access-restricted posts and communicate safe handling of responses and generated reply contexts.

Written by the indexing model from the issue text.