Modify an index's model [ENHANCEMENT]

[pandu-k]

Is your feature request related to a problem? Please describe.
There is no way to update an index's model

Describe the solution you'd like
An advanced feature: update an index's model

Describe alternatives you've considered
Reindex the index. For custom models: change the model URL to point to a new model.

Additional considerations

1. Understand that this operation is eventually consistent, at best
   • Users need to be aware of this
   • Add_docs calls concurrent to model modifying may be ambiguous as to what model generated that vector
2. The new model loading process. Perhaps it is something like this:
   • The index cache should be regularly refreshing (double check this)
   • When a new model is detected, eject the currently loaded model
   • Then, load in the new model
3. How do we validate dimensions, for example?
   • Can we prevent users from breaking their own index?
   • If they do break their own index, can users easily tell what went wrong? Can they easily correct this?
4. What about overloading and breaking the index?
   • What happens if a user tries to continually change the models?
5. What about the vector <> model relationship
   • It would be nice to figure out what model a certain vector was generated by, but we should sketch out possible solutions and feasibility for this