Add a "metadata" field.

[Stebalien]

All files/directories should support metadata so this should be safe as a common field.

[mikeal]

There seemed to be some disagreement in prior threads as to what should go in the “metadata” field and what should not. Is there a reason we shouldn’t just push metadata into the unclaimed top level fields?

[Stebalien]

Not really. I mean, my primary worry is that we may want to add a new "reserved" top-level field but, really, that's probably not an issue (we're going to be doing never-ending duck-typing anyways).