[5.x] Allow asset container order to be specified

[ryanmitchell]

Since https://github.com/statamic/cms/pull/4947 asset containers have been ordered by title, which is mostly a good thing.

However sometimes, you have an asset container that is low priority and rarely used, but then appears first, which isnt so good. A small quality of life improvement is to let the order by specified, while still falling back to title when its not.

This PR introduces an order key to the yaml, which the CP then sorts by when outputting the list, eg order: 99

Before:

After: