amari.kpi: Add support for DRB.UEActive measurement

Expand upon the new data models introduced in https://lab.nexedi.com/kirr/xlte/-/merge_requests/9 by teaching the amari.kpi driver to populate the DRB.UEActive Measurement field.

Mirroring the kpi module, some additional infrastructure is added to the driver in order to support statistical profiles, even though it is only the necessary per-cell measurement extraction utilities for now. The rest can be comfortably added as they will be needed in the future.