Probably should upgrade the parsing to an IDL while we're at it. Update: or not, because that actually sounds like a potential black hole given how much we pass around BSON elsewhere in the index building layer.

// Defaults to false in-memory if absent
runTwoPhaseIndexBuild: <bool>,

// Defaults to "scanning" if absent. Can only be set to "scanning", "verifying" or "committing"
buildPhase: <string>,

// Defaults to 1 if absent.
versionOfBuild: <integer>,

// No default if absent. Should have a bool function to say whether it is present.
buildConstraintViolationsFile: <string>,

newWritesInterceptorTable: <string> (pick a field name that seems suitable for this)

Check in with the design before finalizing.