Add table information_schema.tidb_index_usage to show the usage statistics of an index

[YangKeao]

Enhancement

Most of the necessary statistic data has been collected in the RuntimeStatsColl. We can collect them into a global structure to show the usages of each index.

• Add system variable tidb_enable_object_stats to describe whether to enable collecting index usage. #49831
• Add an RFC for index usage. #49917
• Modify the existing SessionIndexUsageCollector for new index usage design #49835
• Add GC of dropped schema/table/index
• Add a statement index usage collector to deduplicate index for query count. #50121
• Record the index usages at each related executors. #50261
• Construct the table information_schema.tidb_index_usage and information_schema.cluster_tidb_index_usage to show the statistics information. #50823
• Add view sys.schema_unused_index #50886
• Remove unused codes from the previous implementation

[bb7133]

Could you please add a RFC for this?