Allow wildcard for URL source plugin

Made a MR with a few small fixes.

re: #28, allowing callbacks might be useful too, but globs are a useful tool that we have in a filesystem which we don't have for URLs, which make a declarative definition possible. So I think it makes sense to have them too.

Found a problem when the URL doesn't use a stream wrapper like public:// and is intead an absolute filepath.

Working on fixing it...

re #28 again... ah now I've read more of this patch and the patch at the other issue, I see how they could work together.

✨ Allow callback for Url source, and single item Json plugin Needs work should be changed so it defers executing the callback, and that's when this MR should run the globbing too -- in neither case should we be spookily changing the plugin's configuration, that's just confusing DX!