Collect index statistics in a system table

[Ngalstyan4]

It would be good to have a GUC-gated flag that enables statistics collection on index usage patterns in a systems table (e.g. table with schema lantern.index_usage_statistics(indexoid OID, tableoid OID, indexdef TEXT, ...metrics);
In particular, it would be good to collect the following for each index:

• Average number of graph nodes traversed per search
• Average memory usage per search
• Read amplification (number of bytes read from DB index pages over number of bytes necessary for hnsw graph traversal)