[BugFix] Clean Up Nominal GDP & Add EconDB As Provider

[deeleeramone]

1. Why?:

   • OECD was downloading unnecessary data that contributed significantly to a slow response time and wasted memory.

   • Additional providers can be added.

2. What?:

   • Refactor request to download only requested countries, as data-only, from OECD.
   • Reconfigure OECD params as: frequency, units, price_base.
   • No additional params for EconDB, but real growth metrics (yoy/qoq) are included in the response.
   • Multiple items allowed for country.
   • All GDP values (not units = index) are in $USD, have been expanded to their full length, and are directly comparable between countries.

3. Impact:

   • Improved response time.
   • "units" param is now: ["level", "index", "capita"]
   • EconDB added as provider to economy.gdp.nominal

4. Testing Done:

   • Integration/Unit tests.

There are notable differences in the values returned from OECD vs. EconDB, factors related to OECD's applied normalization and annualized transformation. The results from EconDB more closely resemble other sites' "GDP by Country" rankings.