Feature Request: Add DB::ApproximateKeyAnchors

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: https://github.com/facebook/mysql-5.6/issues/1245

Start by reading TableReader::ApproximateKeyAnchors and the existing DB API entry points. Determine how the sampling result should be exposed for applications such as MyRocks to partition input data for parallel processing. Done means DB provides this capability with behavior consistent with the existing TableReader function.

Written by the indexing model from the issue text.