add time_period_freq file attribute

According to https://www.cesm.ucar.edu/models/cesm2/naming-conventions#modelOutputFilenames, CESM output should include a file attribute `time_period_freq` that indicates the frequency of time sampling (I think whether the output is time-averaged or instantaneous). History files written by MOM6/FMS do not currently have this metadata. I haven't looked into how to implement this, including if an implementation belongs within FMS or within MOM6. I'm guessing FMS, so I'm filing the issue here.