Return array of keys in storage

I was looking to use this on another project and I needed to get the list of keys in storage. This seemed like a core piece of functionality rather than writing a plugin for it.
Here, `concat` is not right. Basil allows you to store the same key/value couple into different storages. You may have multiple key duplicates in your array.

Interesting feature here could be to return not an array but an object with found keys and in which storages there are present (eg `{key1: ["local", "cookie"], key2: ["session"]}`. This solution might be more difficult to handle but might also solve previous duplicate issue (by also giving more (useful?) info) ;)

Finally, this is here only in this method that we should filter by namespace.

What do you think about that ?

Best