A new unique index format will be introduced. To allow selection of this new format while creating a unique index, add a gating variable in MongoDB via a global conditional. Identify the places in code that do unique index work with WiredTiger and stub that code to deal with new format.
No need to plumb it through to a public configurable option.

Also add a new IndexDescriptor::IndexVersion flag kV2Unique, which will be set on index creation if the new index format is selected by enabling the gating variable.