Remove all traces of the key generic when in the context of search indexes

[tishun]

Feature Request

Is your feature request related to a problem? Please describe

As a followup to #3456 we should also consider the return types.
Specifically the ftList() and the generics in most read methods.

Describe the solution you'd like

Index names should not be treated as keys.

Describe alternatives you've considered

N/A

Teachability, Documentation, Adoption, Migration Strategy

N/A