Feature Request: Add DB::ApproximateKeyAnchors

[rockeet]

This feature request is picked from facebook/rocksdb#10888

Now RocksDB has TableReader::ApproximateKeyAnchors for sampling key boundaries for sub compaction.

It is better to expose ApproximateKeyAnchors to DB for applications, such as:

In MyRocks, ddl operations such as create index, can using this function to partition the input data to processing with multi threads. (InnoDB has innodb_ddl_threads for this purpose)

I had filed an Feature Request for MyRocks about this feature: facebook/mysql-5.6#1245