Add support for local secondary indexes

[kopatsy]

Because attrtibute are ignored on the aws_dynamodb_table resource, one will not be able to create a local secondary index with attributes that are not already declared by an existing GSI or the table itself.
The proposed workaround is to introduce a gsi_local_secondary_index resource in this provider.

[schampilomatis]

You can only create local secondary indexes when a table is created, meaning that this is a non-issue.
From https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/LSI.html
Local secondary indexes on a table are created when the table is created.