Improve SAI index type handling

There are now 3 different index types supported by SAI :- literal, numeric & vector. These each have different in-memory, on-disk & search requirements.

Currently, the different types are supported using conditional statements throughout the SAI codebase.

It would be better if each index type was represented by a strategy that provided the non-specific SAI code with the support needed to handle the index type.

This would make it far easier to add additional index types in the future.