[I/O] DynamoDB: Load data from tables using single-table design

## About
Loading data from DynamoDB into CrateDB may need special treatment compared with other databases, because DynamoDB's relational design paradigms are special.

## Details
According to the blog post [Single-table vs. multi-table design in Amazon DynamoDB](https://aws.amazon.com/blogs/database/single-table-vs-multi-table-design-in-amazon-dynamodb/):

> For people learning about [Amazon DynamoDB](https://docs.aws.amazon.com/dynamodb), the idea of [single-table design](https://www.alexdebrie.com/posts/dynamodb-single-table/) is one of the most mind-bending concepts out there. Rather than the relational notion of having a table per entity, **DynamoDB tables often include multiple different entities in a single table**.

> DynamoDB isn’t a relational database and you shouldn’t use it like one. The learning curve feels steep, but there are really only three or four key concepts you need to learn, and everything else flows from that. Once you understand these basics, you’ll be able to make more informed decisions about how many tables to use in your application.

## Thoughts

Based on reports by others

> every row has a unique schema based on the "type" of the record

we think it might make sense to dispatch records to different target tables in CrateDB instead, based on this information.