staff race_ethnicity

[rlittle08]

Depends on edanalytics/edu_edfi_source#62

Adds dim_staff.race_ethnicity, created via new build model.

Within the build model, the var() references like this:
then '{{ var("edu:staff_demos:hispanic_latino_code", var("edu:stu_demos:hispanic_latino_code")) }}'
introduce new optional variables:

• edu:staff_demos:hispanic_latino_code
• edu:staff_demos:multiple_races_code
• edu:staff_demos:race_unknown
  But if these are not defined, the code defaults to using the edu:stu_demos versions

[sleblanc23]

@rlittle08 I think it would be helpful to add the race_ethnicity column and a description of how it was generated to dim_staff.yml