Add `Index_UpdateVector` function to the LLAPI

[ashtul]

The function will have 3 return values.

1. Old_id and vector match.
2. Old_id matches, vector does not match.
3. Old_id does not match regardless of the vector match.

The function will be used by RediSearch when a document gets updated in order to avoid deletion and addition of a vector which are costly.

Suggestion for a function signature:
typedef int (*Index_UpdateVector)(VecSimIndex* index, const void* blob, size_t old_id, size_t new_id);