Add support for LSI/GSI to EntityManager.findOne()

[polerin]

Currently EntityManager.find() allows for querying against an LSI/GSI by specifying the desired index in the queryOptions. As getting a single item by LSI is a common use case in our consuming code, it would be much easier if this method supported it.

One potential wrinkle for this request is that findOne() currently requires the primary key, and in this instance the sort key is going to be unknown (hence the index query). findOne() signature might need to be updated to PrimaryKey | PartitionKey based on queryOptions.index !== undefined.

[cckn]

Typedorm should not support findOne() for LSI/GSI because DynamoDB does not allow single-item retrieval using these indexes. The findOne() method is optimized for primary key queries, and including index support would conflict with DynamoDB's inherent functionality and potentially confuse users.

The GetItem and BatchGetItem operations can't be used on a local secondary index.

• ref: Local Secondary Indexes - Amazon DynamoDB

The GetItem and BatchGetItem operations can't be used on a global secondary index.

• ref: Using Global Secondary Indexes in DynamoDB - Amazon DynamoDB