[Feature] Add Central Bank Holdings (Federal Reserve)

[deeleeramone]

1. Why?:

   • Expand breadth of coverage.
   • A more complete view of the Federal Reserve's domestic securities holdings.

2. What?:

   • Adds endpoint to Economy router: central_bank_holdings
   • Adds standard model, central_bank_holdings.
   • Adds data from openbb-federal-reserve provider.

3. Impact:

   • Not a breaking change.
   • Additional maintenance, if required.

4. Testing Done:

   • Integration/Unit tests.
   • CLI

5. Reviewer Notes:

   • Try a variety of parameters and combinations.