Following the Engineering Proposal: More Efficient Replication for Container Writes, and utilizing any necessarily added interfaces in SERVER-130645, we should add support to take advantage of this new format when performing writes in the index build's bulk load phase.  This specifically means adding support in the BulkBuilderImpl::_addKeyForCommit(..) for a key-only array of inserts.

Note that there may be a few caveats here:

• The value used in the container_write::insert(..) is key.getTypeBitsView(), this may or may not be empty and needs to be handled properly so that the optimized path can be used.
• We need to ensure there are no issues with idempotency (ErrorCodes::KeyExists / _nonexistentKeyGuarantee), though this should be handled by the recently completed SERVER-129824.