[IMP] allow group_operator on any field

There's really no reason to limit group_operator to numeric fields and Postgres's flexible aggregate make the ability to aggregate on non-numeric fields (and/or non-numerically)

* moves group_operator to Field
* adds a default value of `sum` on integer, float and monetary fields
* in read_group, filters on fields with a group operator rather than by
  type (also allow id fields, since Id doesn't have a group_operator by
  default it will be ignored anyway) (sequence is still specifically
  excluded for now because they're usually regular Integer fields)

Closes #12372

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