`load_indices` should include index config

Ideally the output from load_indices would allow to both know

• Whether the index is an IvfPq index or a Btree index
• What parameters were used to train the index

Today I can kind of guess the first one based on the type of column but once we add more vector index types this will no longer be possible.

I have no way today of getting the parameters. This can be very useful because users may forget these things and want to examine them (e.g. because they've learned more about vector indices and now they want to know if they need to rebuild their index or not).