Retrieve GSA per diem rates for federal travel using the GSA API

Steps

1. GET https://api.gsa.gov/travel/perdiem/v2/rates/city/{city}/state/{stateAbbrev}/year/{year}?api_key=<API_KEY> to retrieve lodging and M&IE (meals and incidental expenses) per diem rates for a specific city, state, and fiscal year
2. Alternatively, GET by ZIP code via https://api.gsa.gov/travel/perdiem/v2/rates/zip/{zip}/year/{year}?api_key=<API_KEY> for address-level lookups
3. Parse the response 'request' array for 'months' data, noting that lodging rates vary by month within a fiscal year for seasonal destinations while M&IE rates are typically fixed throughout the year
4. For locations not found by city/state (returning 'rates not found'), fall back to the standard CONUS rate by requesting the GSA standard rate or checking the response field 'isStandardRate': true
5. Cache rates by city/state/year as they are set once per federal fiscal year (October 1) and do not change during the year