This introduces new syntax for the createIndex() command, behind a feature flag.

We may want to wait on any docs/drivers changes until after the feature flag is removed, but I'm marking the "teams impacted" box just to be safe.

The new syntax is described in this document: https://docs.google.com/document/d/1RsGAKKHtT01KoBPYV47h0x0FR0Fc_Jtn2skigpHcN4A/edit

Description of Linked Ticket

Should be able to create the index, read it in the listIndexes output, and still see it after a restart. Any usage of it should uassert, though we don't expect to hit those - just to make sure the fuzzer doesn't fail too much.