swift: add support for max_keys when retrieving dir items

[plribeiro3000]

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Currently there is no remapping in getting directory keys. This means that people are forced to add 'limit' explicitly into the calls. This is a problem when you try to have code that's compatible with multiple cloud vendors, as this breaks some other APIs, like Google Cloud Storage.

Raised PR, but it needs a bit more work. I am opening this issue so that someone else can take this over, as I have no SWIFT currently to test against.

Original issue opened by @mtekel at fog/fog#3723

[plribeiro3000]

cc/ @mtekel @geemus @Ladas

[mtekel]

Thanks for raising,

the CF project is waiting for this feature to unblock Google Cloud Storage support (which is broken by this). See https://github.com/cloudfoundry/cloud_controller_ng/pull/452