consider supporting an `enable_approxscan` option

Problem

problem When monitoring the recall of a vector query, you sometimes want to disable all approximate indexes to get the ground_truth and monitor recall. Yet `enable_indexscan` isn't ideal since it turns off all indexes, causing problems in complex queries that make use of multiple tables/indexes. solution Introduce a new guc convention, that if `enable_approxscan` is set and marked as off, turns off approximate indexes (by setting the cost really high as other enable_* gucs do). This guc wouldn't be defined in pgvector, but pgvector would check if it's set (just by name) and act accordingly. I propose `enable_approxscan` as the name but open to other suggestions. I think this convention would ideally, in time apply to all approximate Postgres indexes, certainly we'd also implement it in pgvectorscale's diskann. question Curious about thoughts on whether or not this is a good idea.