Add `/.well-known/nodeinfo` endpoint

*This issue was originally created by [**@6543**](https://github.com/6543) at .*

add the nodeinfo endpoint (http://nodeinfo.diaspora.software/)

and add setting to metrics to specify if user statistics should be public or hidden.
if hidden just report the basic information (software, version, ...)

if public, report back total & active users

Start by locating Dendrite's HTTP route registration and the metrics configuration described in the issue. Check the Nodeinfo specification for the required basic fields and user statistics behavior. Done means /.well-known/nodeinfo serves basic software information, with user totals included only when the new setting makes them public.

Written by the indexing model from the issue text.