Update experimental endpoints to support MV features and null value features

[matthewelwell]

Currently, the 'experimental' endpoints as defined here do not support multivariate flags.

In order to have the FE dashboard migrate to using one or both of these endpoints, we'll need to add support for multivariate flags.

Blocks #7641

[kyle-ssg]

Mentioned here

[gagantrivedi]

Related: #7851