provide various information about vector indexes

Either status variables or new INFORMATION_SCHEMA tables or additional columns in the existing INFORMATION_SCHEMA tables and SHOW commands to show some of

• size of the vector index
• number of deleted rows (can be estimated by number of rows in the hlindex table minus number of rows in the main table)
• actual cache size
• number of layers
• diameter
• subdist related info (M2)
• search stats like (for example)
  • number of distance invocations
  • number of distance_greater_than shortcuts
  • number of jumps per layer

but only values that can actually be used for something