Change StudentBirthCountryDescriptor to use a Type instead

Summary of outcomes for Data Standard 2.1

• Problem Addressed: There is variability in the values loaded for the CountryDescriptor.
• High-level Reasoning: Country was implemented as a descriptor because ISO country codes are prone to frequent change. However, this creates opportunity to vary CountryDescriptor enumerations by implementation, creating the need for vendors to maintain large agency-specific mappings. A recommendation to use ISO types should be documented as a best practice.
• Changes Made: Updated annotation for Country descriptor and all references to Country descriptor to add "It is strongly recommended that entries use only ISO 3166 2-letter country codes."

Original ticket description follows:

Student includes a BirthCountyDescriptor (part of BirthData). This seems to open the possibility that mappings to countries can vary by implementation, creating the need for vendors to maintain large agency-specific mappings.

Instead, we should consider surfacing a Type directly and mapping those countries to a Type.

As an example that sheds light on the ecosystem difficulty, consider: http://tracker.ed-fi.org/browse/ODS-928