add `cds.utils.ms4` to api

[kulliax]

Even though it's not included in the official documentation yet (cds.utils.ms4), is there a way to add this to the API or to the documentation as well?

Reason: Reducing our own conversions and using standard APIs.

[daogrady]

@mariayord
The functionality was first introduced in /cds/pull/5324, seemingly as an internal function as cds.utils._unit.time2ms. It was later renamed and shed the underscore in cds/pull/5548. But honestly, that looks like an accident. Can you please comment on whether this is supposed to be part of the API?