Allow hooks to override metadata

[Murderlon]

Allow hooks to override metadata, only metadata not other Upload properties, in a backwards compatible way to precent breaking changes in the hook design. New metadata should probably be validated as well.

[Acconut]

In tusd, a pre-create hook can return a FileInfoChanges object, which allows the hook to customize a selected set of properties. Maybe this can serve as an inspiration for tus-node-server.