Incorrect ValidationException: No provisioned throughput specified for the table

**Actual Behaviour**

AWS announced support for an automatic provisioning pricing plan. More info: https://medium.com/@softprops/putting-dynamodb-scalability-knobs-on-auto-pilot-3af8520439c9.

In short, you can set `BillingMode: PAY_PER_REQUEST` and remove the ProvisionedThoughput section. But at the moment serverless-dynamodb-local does not support this:

```
Validation Exception -----------------------------------

No provisioned throughput specified for the table
```

**Expected Behaviour**

No validation error

**Steps to reproduce it**

Create a template using `BillingMode: PAY_PER_REQUEST`:
https://gist.githubusercontent.com/softprops/58c60a1ca2a8e97d6ef63bda3d3c8a9f/raw/27d2721f1d1837e3cca00f854810f484f9239ad4/serverless.yml

**Would you like to work on the issue?**

Yes

Start with the linked serverless.yml reproduction template and trace how its BillingMode: PAY_PER_REQUEST table definition is validated by serverless-dynamodb-local. Confirm the reproduction and identify the validation path that produces the provisioned-throughput error; done means the template creates successfully without that validation error.

Written by the indexing model from the issue text.