Add NotRequired things to the DataKey schema

[coretl]

As per bluesky/ophyd-async#137 (comment)

Also removing the Optional from "units" and "precision"

event-model/event_model/documents/event_descriptor.py

Line 59 in fc6e30e

Annotated[Optional[str], Field(description="Engineering units of the value")]

[coretl]

Also choices