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 RTBC 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!