[Feature] Add Mortgage Indices

[deeleeramone]

1. Why?:

   • Expand the data in the fixed income menu.

2. What?:

   • Adds new endpoint: obb.fixedincome.mortgage_indices()
     • Data is from FRED, published by Optimal Blue.
     • Daily indices of US mortgage rates.

3. Impact:

   • No breaking changes.
   • Additional maintenance of endpoints and models.

4. Testing Done:

   • Unit/Integration
   • CLI