S3: deprecate PrefixFolders and introduce HideFolders

[ahouene]

The PrefixFolders option has a behaviour that is confusing and does not allow listing the way other backends do.
This PR deprecates PrefixFolders and introduces another option that helps work with folders in S3: HideFolders.

Its effect is pretty close to that of PrefixFolder, except that instead of just showing folders names without their content as folder/, it will ignore them.

// Example:

s3backend.Store(ctx, "foo", something)
s3backend.Store(ctx, "bar/baz", anything)

// Without HideFolders:
s3backend.List(ctx, "") // "foo", "bar/baz"

// With HideFolders:
s3backend.List(ctx, "") // "foo"
s3backend.List(ctx, "bar/") // "bar/baz"