Bucket alias middleware

[timuralp]

For certain use cases, it may be convenient to be able to rename the backend bucket and specify the mapping via a middleware. This would allow keeping bucket names the same on the application side or avoid collisions.

[gaul]

Agreed that this would be a good middleware, especially if it can do some prefix redirection as well. Might be worth looking at Apache rewrite rules to see what other clever people have thought of (globs?).