IdP metadata config docs

[Cantor, Scott]

> 1. Is anyone using (or has anyone ever used)
> ResourceBackedMetadataProvider? If so, can you briefly describe your use
> case? (There isn't much in the archives about this esoteric metadata provider.)

There were back and forth releases early on that switched from FileBacked to ResourceBacked for reasons I don't really have any history on, but there was a point where we finally just decided to stop doing that. Most people using it are doing it for one of two reasons:

- our defaults used it, or they found some example using it and copied it
- subversion-based metadata

The former use case is officially deprecated (use FileBackedHTTP) and the latter is being deprecated in that particular form because we can't really ship the svn libraries due to licensing and svn is dying anyway so it's not worth maintaining. People can continue to supply their own Resource types for custom use cases if needed, and you can use a new one we built that can actually call out to a script. It's the easy way to implement e.g. a database-backed source. So it's mostly an extension point that handles all the metadata lifting and just relies on a plugin to get the stream.

-- Scott