[Bug fix] - Handle multiple items with arbitrary type

[montezdesousa]

1. Why?

   • Multiple items only support str type
     • fields with type str is converted to Union[str, List[str]]
     • fields with type float/int should be converted to Union[float/int, List[float/int]]

2. What?

   • Use List[<original_type>] instead of List[str]
   • Update obb.fixedincome.corporate.spot_rates accordingly
   • Fixes @handle_exceptions bug when original attribute is not present
   • Removes dangerous default [10.0] from maturity
   • Removes dangerous default ["spot_rate"] from category
   • Use comma separated string for both maturity and category

3. Impact:

   • Arbitrary types allowed for multiple items

4. Testing Done:

• Run integration tests added in this PR

Also,

obb.fixedincome.corporate.spot_rates().to_df()
obb.fixedincome.corporate.spot_rates(maturity=["1", 5.5]).to_df()
obb.fixedincome.corporate.spot_rates(maturity="1,5.5").to_df()
obb.fixedincome.corporate.spot_rates(maturity=5.5).to_df()
obb.fixedincome.corporate.spot_rates(category="spot_rate").to_df()
obb.fixedincome.corporate.spot_rates(category="spot_rate,par_yield").to_df()